Rubber composition for tire tread

ABSTRACT

A rubber composition comprises a diene rubber including not less than 25% by weight of a modified conjugated diene polymer rubber and, per 100 parts by weight of the diene rubber, from 25 to 65 parts by weight of a filler, the filler comprising not less than 50% by weight of silica. The silica has a DBP absorption number of 190 to 250 ml/100 g, an N 2 SA of 194 to 225 m/g, and a CTAB of 185 to 215 m 2 /g. A ratio of the N 2 SA to the CTAB is from 0.9 to 1.4. The modified conjugated diene polymer rubber has a terminal modified group including a functional group that interacts with the silica. In the modified conjugated diene polymer rubber, aromatic vinyl unit content is from 38% to 48% by weight, vinyl unit content is from 20% to 35%, and weight-average molecular weight is from 600,000 to 1,000,000.

TECHNICAL FIELD

The present technology relates to a rubber composition for use in tire treads and particularly relates to a rubber composition for use in tire treads that enhances low rolling resistance and wet performance.

BACKGROUND ART

Increased interest in the global environmental issues has led to a demand for superior fuel consumption performance in pneumatic tires, along with high wet performance and superior safety performance. As a result, by compounding silica in rubber compositions that form tread portions, heat build-up has been suppressed, rolling resistance reduced, and fuel consumption performance improved and, also, dynamic visco-elasticity characteristics of the tread rubber has been improved which has led to enhancements in wet performance. Efforts have also been made to use silica having a small particulate diameter for the purpose of further increasing the effects associated with the compounding of silica. However, silica has poor affinity with diene rubber and dispersibility tends to be insufficient. Particularly, when the particle diameter of the silica is small, dispersibility worsens and, as a result, the effects of improving low rolling resistance and wet performance by modifying dynamic visco-elasticity characteristics such as the loss tangent (tan δ) of the rubber composition and the like have not been achievable. Additionally, reinforcing effects tend to be lower when compounding silica in a rubber component than when compounding carbon black. As a result, there is a problem in that when dispersibility is poor, wear resistance is insufficient.

To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-091498 as well as patent applications WO/2005/021637 and WO/2003/102053 propose improving the dispersibility of silica by compounding silica in a rubber composition with a terminal-modified solution polymerization styrene butadiene rubber where the terminals are modified by a polyorganosiloxane or the like, thereby reducing heat build-up (tan δ at 60° C.) and enhancing wet grip performance (tan δ at 0° C.). However, the low rolling resistance and wet performance obtained by the technology proposed in Patent Documents 1 to 3 is below that demanded by users and there is a need for further improvement in low rolling resistance and wet performance.

SUMMARY

The present technology provides a rubber composition for use in tire treads by which low rolling resistance and wet performance can be enhanced to or beyond conventional levels.

A rubber composition for use in tire treads of the present technology that achieves the objects described above comprises a diene rubber including not less than 25% by weight of a modified conjugated diene polymer rubber and, per 100 parts by weight of the diene rubber, from 25 to 65 parts by weight of a filler. The filler comprises not less than 50% by weight of silica. The silica has a DBP absorption number of 190 to 250 ml/100 g, a nitrogen specific surface area (N₂SA) of 194 to 225 m²/g, and a CTAB specific surface area (CTAB) of 185 to 215 m²/g. A ratio of the N₂SA to the CTAB (N₂SA/CTAB) is from 0.9 to 1.4. Moreover, the modified conjugated diene polymer rubber is obtained by copolymerizing a conjugated diene monomer unit and an aromatic vinyl monomer unit in a hydrocarbon solvent using an organic active metal compound as an initiator. A resulting active conjugated diene polymer chain thereof has a terminal modified group, obtained by reacting the active terminal of the polymer chain with at least one type of compound having a functional group that is reactable with the active terminal of the polymer chain. The terminal modified group has a functional group that interacts with the silica. Furthermore, in the modified conjugated diene polymer rubber, aromatic vinyl unit content is from 38% to 48% by weight, vinyl unit content is from 20% to 35%, and weight-average molecular weight is from 600,000 to 1,000,000.

The rubber composition for use in tire treads of the present technology comprises the diene rubber comprising not less than 25% by weight of the modified conjugated diene polymer rubber in which the aromatic vinyl unit content is from 38% to 48% by weight, the vinyl unit content is from 20% to 35%, and the weight-average molecular weight is from 600,000 to 1,000,000. The active conjugated diene polymer chain is obtained by copolymerizing the conjugated diene monomer unit and the aromatic vinyl monomer unit. The active terminal of the active conjugated diene polymer chain is reacted with at least one type of compound having the functional group that is reactable with the active terminal in order to form the terminal modified group. The terminal modified group has the functional group that interacts with silica. Additionally, the rubber composition for use in tire treads of the present technology comprises, per 100 parts by weight of the diene rubber, from 25 to 65 parts by weight of the filler having a DBP absorption number of 190 to 250 ml/100 g, a nitrogen specific surface area (N₂SA) of 194 to 225 m²/g, and a CTAB specific surface area (CTAB) of 185 to 215 m²/g; the ratio (N₂SA/CTAB) of the N₂SA to the CTAB is from 0.9 to 1.4; and not less than 50% by weight of the filler is silica. As a result, affinity between the diene rubber and the silica is increased and the dispersibility of the silica is enhanced, leading to a reduction in heat build-up, a decline in rolling resistance, and an enhancement in wet performance. Particularly, the modified conjugated diene polymer rubber takes on a fine phase-separated form due to setting the aromatic vinyl unit content to from 38% to 48% by weight. Moreover, the terminal modified group, produced through the reaction of the active terminal of the active conjugated diene polymer chain and the at least one compound having the functional group that is reactable with the active terminal of the active conjugated diene polymer chain, has the functional group that interacts with the silica and the weight-average molecular weight of the modified conjugated diene polymer rubber is set to from 600,000 to 1,000,000, resulting in the concentration of the terminal modified group being made appropriate. Therefore, the terminal modified group acts effectively on the silica, the dispersibility of the silica is further improved, the rolling resistance of the pneumatic tire is significantly reduced, and the wet performance is further enhanced.

With the rubber composition for use in tire treads of the present technology, the diene rubber preferably comprises a natural rubber and/or a butadiene rubber in addition to the modified conjugated diene polymer rubber in order to enhance the wear resistance to or beyond conventional levels. In one embodiment, 100% by weight of the diene rubber preferably comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber and from 15% to 45% by weight of the natural rubber. Thereby, wear resistance can be enhanced while maintaining superior low rolling resistance and wet performance.

In another embodiment, 100% by weight of the diene rubber preferably comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber and from 15% to 35% by weight of the butadiene rubber. Thereby, wear resistance can be enhanced while maintaining superior low rolling resistance and wet performance.

The compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain described above preferably comprises at least one type of polyorganosiloxane compound selected from general formulae (I) to (III) below.

In formula (I), R¹ to R⁸ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons; X¹ and X⁴ are identical or different and are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, alkyl groups having from 1 to 6 carbons, or aryl groups having from 6 to 12 carbons; X² is a group having a functional group that reacts with the active terminal of the active conjugated diene polymer chain; X³ is a group including from 2 to 20 repeating alkylene glycol units, a portion of the X³ moieties optionally being groups derived from groups including from 2 to 20 repeating alkylene glycol units; and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to 200.

In formula (II), R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons; and X⁵ to X⁸ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain.

In formula (III), R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons; and X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain; and s is an integer from 1 to 18.

The low rolling resistance and the wet performance can be enhanced to or beyond conventional levels via a pneumatic tire in which the rubber composition of the present technology is used in the tread portion.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial cross-sectional view in a tire meridian direction illustrating an example of an embodiment of a pneumatic tire in which a rubber composition for use in tire treads of the present technology is used.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of an embodiment of a pneumatic tire in which a rubber composition for use in tire treads is used. In FIG. 1, 1 is a tread portion, 2 is a side wall portion, and 3 is a bead portion.

In FIG. 1, two layers of a carcass layer 4, formed by arranging reinforcing cords extending in a tire radial direction in a tire circumferential direction at a predetermined pitch and embedding these reinforcing cords in a rubber layer, is disposed extending between left and right side bead portions 3. Both ends are made to sandwich a bead filler 6 around a bead core 5 that is embedded in the bead portions 3 and are folded back in a tire axial direction from the inside to the outside. An inner liner layer 7 is disposed inward of the carcass layer 4. Two layers of a belt layer 8, formed by arranging reinforcing cords extending inclined to the tire circumferential direction in the tire axial direction at a predetermined pitch and embedding these reinforcing cords in a rubber layer, is disposed on an outer circumferential side of the carcass layer 4 of the tread portion 1. The reinforcing cords of the two layers of a belt layer 8 cross interlamilarly so that the incline directions with respect to the tire circumferential direction are opposite each other. A belt cover layer 9 is disposed on an outer circumferential side of the belt layers 8. The tread portion 1 is formed from a tread rubber layer 12 on an outer circumferential side of the belt cover layer 9. The tread rubber layer 12 is formed from the rubber composition for use in tire treads. A side rubber layer 13 is disposed outward of the carcass layer 4 in each side wall portion 2, and a rim cushion rubber layer 14 is provided outward of the portion of the carcass layer 4 that is folded back around each of the bead portions 3.

In the rubber composition for use in tire treads of the present technology, the rubber component is a diene rubber and the diene rubber necessarily comprises a modified conjugated diene polymer rubber. The modified conjugated diene polymer rubber is a conjugated diene polymer rubber produced by solution polymerization, configured to have functional groups at both terminals of the molecular chain. By compounding the modified conjugated diene polymer rubber, affinity with silica is increased, and dispersibility is improved. As a result, the effects of the silica are further enhanced and wet performance is improved. Additionally, wear resistance is enhanced in accordance with the enhancement in the dispersibility of the silica in the diene rubber.

In the present technology, the backbone of the modified conjugated diene polymer is formed by a copolymer obtained by copolymerizing a conjugated diene monomer unit and an aromatic vinyl monomer unit. Examples of the conjugated diene monomer unit include 1,3-butadiene, isoprene(2-methyl-1,3-butadiene), 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, 1,3-pentadiene, and the like. Examples of the aromatic vinyl monomer unit include styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, alpha-methylstyrene, 2,4-dimethylstyrene, 2,4-diisoisopropylstyrene, 4-tert-butylstyrene, divinylbenzene, tert-butoxystyrene, vinylbenzyldimethylamine, (4-vinylbenzyl)dimethylaminoethyl ether, N,N-dimethyl aminoethylstyrene, vinyl pyridine, and the like.

The terminals of the conjugated diene polymer backbone are preferably formed from isoprene unit blocks. As a result of the terminals being formed from isoprene unit blocks, when the terminals are modified and the silica is compounded, affinity between the modified conjugated diene polymer and the silica is excellent and reduced heat build-up and wear resistance are also excellent. Thus, in cases where the conjugated diene monomer units forming the polymer comprise conjugated dienes other than isoprene units, isoprene unit blocks are preferably introduced on the polymer terminals by adding isoprene to a solution containing the polymer having an active terminal prior to adding the compound having the functional group that is reactable with the active terminal of the active conjugated diene polymer chain or, alternatively, between subsequent adding of portions of this compound.

In the present technology, the conjugated diene polymer is prepared by copolymerizing the conjugated diene monomer unit and the aromatic vinyl monomer unit described above in a hydrocarbon solvent, using an organic active metal compound as an initiator. It is sufficient that the hydrocarbon solvent be a commonly used solvent, and examples thereof include cyclohexane, n-hexane, benzene, toluene, and the like.

The organic active metal catalyst to be used is preferably an organic alkali metal compound, and examples thereof include organic monolithium compounds such as n-butyllithium, sec-butyllithium, t-butyllithium, hexyl lithium, phenyl lithium, stilbene lithium, and the like; organic polyhydric lithium compounds such as dilithiomethane, 1,4-dilithiobutane, 1,4-dilithio-2-ethylcyclohexane, 1,3,5-trilithiobenzene, and the like; organic sodium compounds such as sodium napthalene and the like; and organic potassium compounds such as potassium napthalene and the like. Additionally, 3,3-(N,N-dimethylamino)-1-propyl lithium, 3-(N,N-diethylamino)-1-propyl lithium, 3-(N,N-dipropylamino)-1-propyl lithium, 3-morpholino-1-propyl lithium, 3-imidazole-1-propyl lithium, and organic lithium compounds having their chains extended by 1 to 10 units of butadiene, isoprene, or styrene; and the like can be used.

In the polymerization reaction, a polar aprotic compound such as an ether such as diethylether, diethylene glycol dimethylether, tetrahydrofuran, 2,2-bis(2-oxolanyl)propane, and the like, or an amine such as triethylamine, tetramethyl ethylenediamine, and the like may also be added for the purpose of randomly copolymerizing the aromatic vinyl monomer units and the conjugated diene monomer units.

In the present technology, at least one type of compound having a reactable functional group is attached to the active terminal of the active conjugated diene polymer chain obtained by copolymerizing the conjugated diene monomer units and the aromatic vinyl monomer units, and, thereby, a terminal modified group is produced. In this case, it is sufficient that the compound having the reactable functional group at the active terminal of the active conjugated diene polymer chain be attached to at least one active conjugated diene polymer chain, and one or more active conjugated diene polymer chains can be attached to each compound. That is, the modified conjugated diene polymer rubber used in the present technology can include modified rubbers having modifying groups at both terminals of the conjugated diene polymer, modified rubbers in which one or more of the modifying groups is optionally attached to a different conjugated diene polymer, and mixtures of a plurality of these modified rubbers. Additionally, the reaction between the active terminal of the active conjugated diene polymer chain and the compound having the functional group that is reactable with the active terminal can be a single-stage or multiple-stage reaction. Moreover, an identical or different compound may be sequentially reacted.

In the present technology, examples of the compound having the functional group that is reactable with the active terminal of the active conjugated diene polymer chain include tin compounds, silicon compounds, silane compounds, amido compounds and/or imide compounds, isocyanate and/or isothiocyanate compounds, ketone compounds, ester compounds, vinyl compounds, oxirane compounds, thiirane compounds, oxetane compounds, polysulfide compounds, polysiloxane compounds, polyorganosiloxane compounds, polyether compounds, polyene compounds, halogen compounds, and compounds having fullerenes. Among these, polyorganosiloxane compounds are preferable. One of these compounds or combinations of a plurality of these compounds can be attached to the polymer.

Specific examples of the compound that is reactable with the active terminal of the active conjugated diene polymer chain include polyglycidyl ethers of polyhydric alcohol such as ethylene glycol diglycidyl ether, glycerin triglycidyl ether, and the like; polyglycidyl ethers of aromatic compounds having two or more phenol groups such as bisphenol A diglycidylate and the like; polyepoxy compounds such as 1,4-diglycidyl benzene, 1,3,5-triglycidyl benzene, liquid polybutadiene polyepoxydate, and the like; epoxy group-containing tertiary amines such as 4,4′-diglycidyl-diphenyl methylamine, 4,4′-diglycidyl-dibenzyl methylamine, and the like; diglycidyl amino compounds such as diglycidyl aniline, diglycidyl-o-toluidine, tetraglycidyl metaxylylene diamine, tetraglycidyl amino diphenylmethane, tetraglycidyl-p-phenylenediamine, diglycidyl amino methylcyclohexane, tetraglycidyl-1,3-bis amino methylcyclohexane, and the like; and the like.

Examples of the silicon compound include tetrachlorosilicon, tetrabromosilicon, methyltrichlorosilicon, butyltrichlorosilicon, dichlorosilicon, bis(trichlorosilyl)silicon, and the like.

Examples of the tin compound include tetrachlorostannate, tetrabromostannate, methyltrichlorostannate, butyltrichlorostannate, dichlorostannate, bis(trichlorosilyl)stannate, and the like.

Examples of the silane compound include silane compounds having at least one selected from an alkoxy group, a phenoxy group, and a halogen. Examples of such silane compounds include dimethoxy dimethylsilane, diphenoxy dimethylsilane, diethoxy diethylsilane, triphenoxy methylsilane, triphenoxy vinylsilane, trimethoxy vinylsilane, triethoxy vinylsilane, tri(2-methylbutoxy)ethylsilane, tri(2-methylbutoxy)vinylsilane, triphenoxy phenylsilane, tetraphenoxysilane, tetraethoxysilane, tetramethoxysilane, tetrakis(2-ethylhexyloxy)silane, phenoxydivinyl chlorosilane, methoxybiethyl chlorosilane, diphenoxymethyl chlorosilane, diphenoxyphenyl iodosilane, diethoxymethyl chlorosilane, dimethoxymethyl chlorosilane, trimethoxy chlorosilane, triethoxy chlorosilane, triphenoxy chlorosilane, tris(2-ethylhexyloxy)chlorosilane, phenoxymethyl dichlorosilane, methoxyethyl dichlorosilane, ethoxymethyl dichlorosilane, phenoxyphenyl diiodosilane, diphenoxy dichlorosilane, dimethoxy dichlorosilane, bis(2-methylbutoxy)dibromosilane, bis(2-methylbutoxy)dichlorosilane, diethoxy dichlorosilane, methoxy trichlorosilane, ethoxy trichlorosilane, phenoxy trichlorosilane, (2-ethylhexyloxy)trichlorosilane, (2-methylbutoxy)trichlorosilane, and the like.

Additionally, aside from the functional groups described above, the silane compound can have a glycidyl group, an epoxy group, a methacryloxy group, and the like. Examples of such silane compounds include γ-glycidoxyethyl trimethoxysilane, γ-glycidoxypropyl trimethoxysilane, γ-glycidoxybutyl trimethoxysilane, γ-glycidoxypropyl triethoxysilane, γ-glycidoxypropyl tripropoxysilane, γ-glycidoxypropyl tributoxysilane, γ-glycidoxypropyl triphenoxysilane, γ-glycidoxypropyl methyldimethoxysilane, γ-glycidoxypropyl ethyldimethoxysilane, γ-glycidoxypropyl ethyldiethoxysilane, γ-glycidoxypropyl methyldiethoxysilane, γ-glycidoxypropyl methyldipropoxysilane, γ-glycidoxypropyl methyldibutoxysilane, γ-glycidoxypropyl methyldiphenoxysilane, γ-glycidoxypropyl dimethylmethoxysilane, γ-glycidoxypropyl diethylethoxysilane, γ-glycidoxypropyl dimethylethoxysilane, γ-glycidoxypropyl dimethylphenoxysilane, γ-glycidoxypropyl diethylmethoxysilane, γ-glycidoxypropyl methyldiisopropeneoxysilane, bis(γ-glycidoxypropyl)dimethoxysilane, bis(γ-glycidoxypropyl)diethoxysilane, bis(γ-glycidoxypropyl)dipropoxysilane, bis(γ-glycidoxypropyl)dibutoxysilane, bis(γ-glycidoxypropyl)diphenoxysilane, bis(γ-glycidoxypropyl)methylmethoxysilane, bis(γ-glycidoxypropyl)methylethoxysilane, bis(γ-glycidoxypropyl)methylpropoxysilane, bis(γ-glycidoxypropyl)methylbutoxysilane, bis(γ-glycidoxypropyl)methylphenoxysilane, tris(γ-glycidoxypropyl)methoxysilane, γ-methacryloxypropyl trimethoxysilane, γ-methacryloxypropyl triethoxysilane, γ-methacryloxymethyl trimethoxysilane, γ-methacryloxyethyl triethoxysilane, bis(γ-methacryloxypropyl)dimethoxysilane, tris(γ-methacryloxypropyl)methoxysilane, β-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-triethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-tripropoxysilane, β-(3,4-epoxycyclohexyl)ethyl-tributoxysilane, β-(3,4-epoxycyclohexyl)ethyl-triphenoxysilane, β-(3,4-epoxycyclohexyl)propyl-trimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-methyldimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-ethyldimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-ethyldiethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-methyldiethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-methyldipropoxysilane, β-(3,4-epoxycyclohexyl)ethyl-methyldibutoxysilane, β-(3,4-epoxycyclohexyl)ethyl-methyldiphenoxysilane, β-(3,4-epoxycyclohexyl)ethyl-dimethylmethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-diethylethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-dimethylethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-dimethylpropoxysilane, β-(3,4-epoxycyclohexyl)ethyl-dimethylbutoxysilane, β-(3,4-epoxycyclohexyl)ethyl-dimethylphenoxysilane, β-(3,4-epoxycyclohexyl)ethyl-diethylmethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-methyldiisopropeneoxysilane, and the like.

Examples of the isocyanate compound or isothiocyanate compound include aromatic polyisocyanate compounds such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, tolidine diisocyanate, triphenylmethane triisocyanate, p-phenylene diisocyanate, tris(isocyanatephenyl)thiophosphate, xylylene diisocyanate, benzene-1,2,4-triisocyanate, naphthalene-1,2,5,7-tetraisocyanate, naphthalene-1,3,7-triisocyanate, phenylisocyanate, hexamethylene diisocyanate, methylcyclohexane diisocyanate, phenyl-1,4-diisothiocyanate, 2,4-tolylene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, and the like.

Further examples include N-substituted aminoketones such as 4-dimethylamino benzophenone, 4-diethylamino benzophenone, 4-di-t-butylamino benzophenone, 4-diphenylamino benzophenone, 4,4′-bis(dimethylamino)benzophenone, 4,4′-bis(diethylamino)benzophenone, 4,4′-bis(di-t-butylamino)benzophenone, 4,4′-bis(diphenylamino)benzophenone, 4,4′-bis(divinylamino)benzophenone, 4-dimethylamino acetophenone, 4-diethylamino acetophenone, 1,3-bis(diphenylamino)-2-propanone, 1,7-bis(methylethylamino)-4-heptanone, and the like and corresponding N-substituted aminothioketones; N-substituted aminoaldehydes such as 4-diethylamino benzaldehyde, 4-divinylamino benzaldehyde, and the like and corresponding N-substituted aminothioaldehydes; N-substituted lactams such as N-methyl-β-propiolactam, N-t-butyl-β-propiolactam, N-phenyl-β-propiolactam, N-methoxyphenyl-β-propiolactam, N-naphthyl-β-propiolactam, N-methyl-2-pyrrolidone, N-t-butyl-2-pyrrolidone, N-phenyl-pyrrolidone, N-methoxyphenyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, N-benzyl-2-pyrrolidone, N-naphthyl-2-pyrrolidone, N-methyl-5-methyl-2-pyrrolidone, N-methyl-3,3′-dimethyl-2-pyrrolidone, N-t-butyl-3,3′-dimethyl-2-pyrrolidone, N-phenyl-3,3′-dimethyl-2-pyrrolidone, N-methyl-2-piperidone, N-t-butyl-2-piperidone, N-phenyl-piperidone, N-methoxyphenyl-2-piperidone, N-vinyl-2-piperidone, N-benzyl-2-piperidone, N-naphthyl-2-piperidone, N-methyl-3,3′-dimethyl-2-piperidone, N-phenyl-3,3′-dimethyl-2-piperidone, N-methyl-ε-caprolactam, N-phenyl-ε-caprolactam, N-methoxyphenyl-ε-caprolactam, N-vinyl-ε-caprolactam, N-benzyl-ε-caprolactam, N-naphthyl-ε-caprolactam, N-methyl-ω-laurilolactam, N-phenyl-ω-laurilolactam, N-t-butyl-laurilolactam, N-vinyl-ω-laurilolactam, N-benzyl-ω-laurilolactam, and the like and corresponding thiolactams; N-substituted ethyleneureas such as 3-dimethyl-2-imidazolidinone, 1,3-diethyl-2-imidazolidinone, 1,3-dipropyl-2-imidazolidinone, 1-methyl-3-ethyl-2-imidazolidinone, 1-methyl-3-propyl-2-imidazolidinone, 1-methyl-3-methyl-2-imidazolidinone, 1-methyl-3-2-ethoxyethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydropyrimidinone, and the like and corresponding N-substituted thioethyleneureas and the like; benzophenones and thiobenzophenones having at least one amino group, alkylamino group or dialkylamino group on one or both benzene rings such as 4,4′-bis(dimethylamino)-benzophenone, 4,4′-bis(diethylamino)-benzophenone, 4,4′-bis(dibutylamino)-benzophenone, 4,4′-diamino benzophenone, 4-dimethylamino benzophenone, and the like and corresponding thiobenzophenones and the like; and the like.

The halogen and/or alkoxy group-containing silicon compound preferably is a compound expressed by general formula (IV) below. A plurality of active conjugated diene polymer chains can easily be attached to a molecule of this compound. Formula (IV) SiX¹ _(p)R¹ _(3-p)

A¹-A³-A²

_(n)SiX² _(q)R² _(3-q)  (IV)

In formula (IV), X¹ and X² are halogen atoms or alkoxy groups having from 1 to 20 carbons. p and q are each independently integers from 0 to 3 and the total number halogen atoms and alkoxy groups having from 1 to 20 carbons in the compound expressed by formula (IV) is not less than 5. R¹ and R² are each monovalent hydrocarbon groups having from 1 to 20 carbons. n is an integer of from 0 to 20 and A¹ and A² are each independently divalent hydrocarbons having a single bond or from 1 to 20 carbons. A³ is a divalent group expressed by the formula —(SiX³ _(r)R³ _(2-r))_(m)—, —NR⁴—, or —N(-A⁴-SiX⁴ _(S)R⁵ _(3-S))—. X³ and X⁴ are halogen atoms or alkoxy groups having from 1 to 20 carbons. R³ and R⁵ are monovalent hydrocarbon groups having from 1 to 20 carbons. R⁴ is a hydrogen atom or a monovalent hydrocarbon group having from 1 to 20 carbons. A⁴ is a divalent hydrocarbon group having a single bond or from 1 to 20 carbons. r is an integer of from 0 to 2 and m is an integer of from 0 to 20. s is an integer of from 0 to 3.

Examples of the compound expressed by general formula (IV) include halogenated silicon compounds such as hexachlorodisilane, bis(trichlorosilyl)methane, 1,2-bis(trichlorosilyl)ethane, 1,3-bis(trichlorosilyl)propane, 1,4-bis(trichlorosilyl)butane, 1,5-bis(trichlorosilyl)pentane, 1,6-bis(trichlorosilyl)hexane, and the like; alkoxysilane compounds such as hexamethoxydisilane, hexaethoxydisilane, bis(trimethoxysilyl)methane, bis(triethoxysilyl)methane, bis(trimethoxysilyl)ethane, bis(triethoxysilyl)ethane, bis(trimethoxysilyl)propane, bis(triethoxysilyl)propane, bis(trimethoxysilyl)butane, bis(triethoxysilyl)butane, bis(trimethoxysilyl)heptane, bis(triethoxysilyl)heptane, bis(trimethoxysilyl)hexane, bis(triethoxysilyl)hexane, bis(trimethoxysilyl)benzene, bis(triethoxysilyl)benzene, bis(trimethoxysilyl)cyclohexane, bis(triethoxysilyl)cyclohexane, bis(triethoxysilyl)benzene, bis(trimethoxysilyl)octane, bis(triethoxysilyl)octane, bis(trimethoxysilyl)nonane, bis(triethoxysilyl)nonane, bis(trimethoxysilyl)ethylene, bis(triethoxysilyl)ethylene, bis(trimethoxysilylethyl)benzene, bis(triethoxysilylethyl)benzene, bis(3-trimethoxysilylpropyl)ethane, bis(3-triethoxysilylpropyl)ethane, and the like; alkoxysilane compounds having an amino group such as bis(3-trimethoxysilylpropyl)methylamine, bis(3-triethoxysilylpropyl)methylamine, bis(3-trimethoxysilylpropyl)ethylamine, bis(3-triethoxysilylpropyl)ethylamine, bis(3-trimethoxysilylpropyl)propylamine, bis(3-triethoxysilylpropyl)propylamine, bis(3-trimethoxysilylpropyl)butylamine, bis(3-triethoxysilylpropyl)butylamine, bis(3-trimethoxysilylpropyl)phenylamine, bis(3-triethoxysilylpropyl)phenylamine, bis(3-trimethoxysilylpropyl)benzylamine, bis(3-triethoxysilylpropyl)benzylamine, bis(trimethoxysilylmethyl)methylamine, bis(triethoxysilylmethyl)methylamine, bis(2-trimethoxysilylethyl)methylamine, bis(2-triethoxysilylethyl)methylamine, bis(triethoxysilylmethyl)propylamine, bis(2-triethoxysilylethyl)propylamine, and the like; alkoxysilane compounds having an amino group such as tris(trimethoxysilylmethyl)amine, tris(2-triethoxysilylethyl)amine, tris(3-trimethoxysilylpropyl)amine, tris(3-triethoxysilylpropyl)amine, and the like; and the like.

The polyorganosiloxane compound is preferably a compound expressed by general formulae (I) to (III) below. That is, the compound having the functional group that is reactable with the active terminal of the active conjugated diene polymer chain preferably includes at least one compound selected from these polyorganosiloxane compounds, and may include a combination of a plurality of these compounds. Additionally, these polyorganosiloxane compounds may be combined with another compound having a functional group that is reactable with the active terminal (e.g. the compound expressed by formula (IV) above).

In formula (I), R¹ to R⁸ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons; X¹ and X⁴ are identical or different and are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, alkyl groups having from 1 to 6 carbons, or aryl groups having from 6 to 12 carbons; X² is a group having a functional group that reacts with the active terminal of the active conjugated diene polymer chain; X³ is a group including from 2 to 20 repeating alkylene glycol units, a portion of the X³ moieties optionally being groups derived from groups including from 2 to 20 repeating alkylene glycol units; and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to 200.

In formula (II), R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons; and X⁵ to X⁸ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain.

In formula (III), R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons; X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain; and s is an integer from 1 to 18.

Examples of the alkyl groups having from 1 to 6 carbons that constitute the R¹ to R⁸, X¹, and X⁴ moieties in the polyorganosiloxane expressed by general formula (I) above include methyl groups, ethyl groups, n-propyl groups, isopropyl groups, butyl groups, pentyl groups, hexyl groups, cyclohexyl groups, and the like. Examples of the aryl groups having from 6 to 12 carbons include phenyl groups, methylphenyl groups, and the like. Among these alkyl groups and aryl groups, methyl groups are particularly preferable.

Examples of the group having the functional group, which reacts with the active terminal of the polymer chain, constituting the X¹, X², and X⁴ moieties in the polyorganosiloxane expressed by general formula (I) include alkoxyl groups having from 1 to 5 carbons, hydrocarbon groups containing a 2-pyrrolidonyl group, and epoxy group-containing groups having from 4 to 12 carbons.

Examples of the alkoxyl groups having from 1 to 5 carbons constituting the X¹, X², and X⁴ moieties include methoxy groups, ethoxy groups, propoxy groups, isopropoxy groups, butoxy groups, and the like. Among these methoxy groups are preferable. In cases where at least one of the X¹, X², and X⁴ moieties is the alkoxyl group having from 1 to 5 carbons, when the polyorganosiloxane having the alkoxyl group is reacted with the active terminal of the active conjugated diene polymer chain, linkage between the silicon atom and the oxygen atom of the alkoxyl group breaks and the active conjugated diene polymer chain attaches directly to the silicon atom, thus forming a single bond.

Preferable examples of the hydrocarbon group containing a 2-pyrrolidonyl group constituting the X¹, X², and X⁴ moieties include the groups expressed by the general formula (V) below.

In formula (V), j is an integer of from 2 to 10, and j is particularly preferably 2.

Thus, when the polyorganosiloxane, in which at least one of the X¹, X², and X⁴ moieties comprises the hydrocarbon group containing the 2-pyrrolidonyl group, is reacted with the active terminal of the active conjugated diene polymer chain, the carbon-oxygen bond in the carbonyl group constituting the 2-pyrrolidonyl group breaks and a structure is formed in which the polymer chain is bonded to the carbon atom.

Preferable examples of the epoxy group-containing group having from 4 to 12 carbons constituting the X¹, X², and X⁴ moieties include the groups expressed by the general formula (VI) below. ZYE  General formula (VI):

In formula (VI), Z is an alkylene group or an alkyl arylene group having from 1 to 10 carbons; Y is a methylene group, a sulfur atom, or an oxygen atom; and E is an epoxy group-containing hydrocarbon group having from 2 to 10 carbons. Among these, preferably Y is an oxygen atom; more preferably Y is an oxygen atom and E is a glycidyl group; and even more preferably Z is an alkylene group having three carbons, Y is an oxygen atom, and E is a glycidyl group.

In the polyorganosiloxane expressed by general formula (I), in cases where at least one of the X¹, X², and X⁴ moieties is an epoxy group-containing group having from 4 to 12 carbons, when the polyorganosiloxane is reacted with the active terminal of the active conjugated diene polymer chain, the carbon-oxygen bond forming the epoxy ring breaks and a structure is formed in which the polymer chain is bonded to the carbon atom.

In the polyorganosiloxane expressed by general formula (I), of the above, X¹ and X⁴ are preferably epoxy group-containing groups having from 4 to 12 carbons or alkyl group having from 1 to 6 carbons. Additionally, X² is preferably an epoxy group-containing group having from 4 to 12 carbons.

In the polyorganosiloxane expressed by general formula (I), X³ is a group including from 2 to 20 repeating alkylene glycol units. Preferable examples of the group including from 2 to 20 repeating alkylene glycol units include the group expressed by general formula (VII) below.

In formula (VII), t is an integer of from 2 to 20, R¹ is an alkylene group or an alkyl arylene group having from 2 to 10 carbons, R³ is a hydrogen atom or a methyl group, and R² is an alkoxyl group or an aryloxy group having from 1 to 10 carbons. Among these, preferably, t is an integer of from 2 to 8, R¹ is an alkylene group having three carbons, R³ is a hydrogen atom, and R² is a methoxy group.

In the polyorganosiloxane expressed by general formula (II), R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons. X⁵ to X⁸ are groups having functional groups that react with the active terminal of the polymer chain.

In the polyorganosiloxane expressed by general formula (III), R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons. X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the polymer chain. s is an integer from 1 to 18.

In the polyorganosiloxane expressed by general formula (II) and general formula (III) above, the alkyl group having from 1 to 6 carbons, the aryl group having from 6 to 12 carbons, and the group having the functional group that reacts with the active terminal of the polymer chain are synonymous with those recited for the polyorganosiloxane expressed by general formula (I).

Furthermore, the terminal modified group produced as a result of the reaction described above has a functional group that interacts with silica. This functional group that interacts with silica may be the functional group included in the structure of the compound described above. The functional group may also be a functional group that is obtained as a result of the reaction between the compound and the active terminal. The functional group that interacts with silica is not particularly limited, and examples thereof include an alkoxysilyl group, a hydroxyl group (including those having organosiloxane structures), an aldehyde group, a carboxyl group, an amino group, an imino group, an epoxy group, an amido group, a thiol group, an ether group, and the like. Among these, the hydroxyl group (including that having an organosiloxane structure) is preferable. Thus, the terminal modified group includes the functional group that interacts with silica and, therefore, affinity with silica is further enhanced, which leads to significant improvement in dispersibility.

In the present technology, the concentration of the terminal modified group in the modified conjugated diene polymer rubber is determined by its relationship to the weight-average molecular weight (Mw) of the modified conjugated diene polymer rubber. The weight-average molecular weight of the modified conjugated diene polymer rubber is from 600,000 to 1,000,000 and is preferably from 650,000 to 850,000. If the weight-average molecular weight of the modified conjugated diene polymer rubber is less than 600,000, the concentration of the terminal modified group of the modified conjugated diene polymer rubber will increase and, while the dispersibility of the silica will be better, the molecular weight of the polymer itself will be low, which leads to the possibility that the strength and rigidity of the rubber composition will be insufficient. Moreover, the degree of improvement in high-temperature viscoelastic characteristics will be limited. Furthermore, the wear resistance of the rubber composition may decline. If the weight-average molecular weight of the modified conjugated diene polymer rubber exceeds 1,000,000, the concentration of the terminal modified group of the modified conjugated diene polymer rubber will decrease, affinity with the silica will be insufficient, and dispersibility will be negatively affected. As a result, the effects of reducing the rolling resistance will be insufficient. Note that the weight-average molecular weight (Mw) of the modified conjugated diene polymer rubber is measured via gel permeation chromatography (GPC), in terms of standard polystyrene.

An aromatic vinyl unit content in the modified conjugated diene polymer rubber used in the present technology is from 38% to 48% by weight and preferably from 40% to 45% by weight. By configuring the aromatic vinyl unit content in the modified conjugated diene polymer rubber to be within this range, the rigidity and the strength of the rubber composition can be increased and the wet resistance can be further enhanced when the rubber is formed into a pneumatic tire. Additionally, the wear resistance of the rubber composition can be further increased. When compounding a diene rubber other than the modified conjugated diene polymer rubber, the modified conjugated diene polymer rubber takes on a fine phase-separated form from the other diene rubber. As a result, the modified conjugated diene polymer rubber gathers locally in the vicinity of the silica particles and the terminal modified groups act effectively on the silica, which leads to the affinity being further enhanced and the dispersibility of the silica being excellent. If the aromatic vinyl unit content in the modified conjugated diene polymer rubber is less than 38% by weight, the effect of forming the fine phase-separated form from the other diene rubber cannot be sufficiently obtained. Additionally, the effects of increasing the rigidity and the strength of the rubber composition cannot be sufficiently obtained. If the aromatic vinyl unit content in the modified conjugated diene polymer rubber exceeds 48% by weight, the glass transition temperature (Tg) of the conjugated diene polymer rubber will rise, the balance between viscoelastic characteristics will worsen, and it will be difficult to obtain the effects of reducing heat build-up. Moreover, it will be difficult to ensure wear resistance. Note that the aromatic vinyl unit content in the modified conjugated diene polymer rubber is measured using infrared emission spectroscopy (Hampton technique).

In the present technology, a vinyl unit content in the modified conjugated diene polymer rubber is from 20% to 35% by weight and is preferably from 26% to 34% by weight. The glass transition temperature (Tg) of the modified conjugated diene polymer rubber can be made appropriate due to configuring the vinyl unit content in the modified conjugated diene polymer rubber to be from 20% to 35% by weight. Additionally, in this case, the fine phase-separated form of the modified conjugated diene polymer rubber from the other diene rubber can be stabilized. Furthermore, wear resistance can be ensured. If the vinyl unit content in the modified conjugated diene polymer rubber is less than 20% by weight, the Tg of the modified conjugated diene polymer rubber will decrease and the dynamic visco-elasticity characteristic loss tangent (tan δ) at 0° C., which is the indicator of wet performance, will decline. Moreover, in this case, the fine phase-separated form of the modified conjugated diene polymer rubber cannot be stabilized. If the vinyl unit content in the modified conjugated diene polymer rubber exceeds 35% by weight, there is a possibility that vulcanization rate will decline and the strength and the rigidity will decline. Note that the vinyl unit content in the modified conjugated diene polymer rubber is measured using infrared emission spectroscopy (Hampton technique).

The formability/processability of a rubber composition can be enhanced by oil extending the modified conjugated diene polymer rubber. The amount of oil extension is not particularly limited, but is preferably not more than 25 parts by weight per 100 parts by weight of the modified conjugated diene polymer rubber. If the amount of oil extension of the modified conjugated diene polymer rubber exceeds 25 parts by weight, the degree of freedom in formulation design when compounding oils, softeners, tackifiers, and the like in the rubber composition will be limited.

Additionally, the glass transition temperature (Tg) of the modified conjugated diene polymer rubber is not particularly limited, but is preferably configured to be from −30° C. to −15° C. By configuring the Tg of the modified conjugated diene polymer rubber to be within this range, the wet performance can be ensured and the rolling resistance can be reduced. The glass transition temperature (Tg) of the modified conjugated diene polymer rubber is measured using a thermograph by differential scanning calorimetry (DSC) at a temperature elevation speed of 20° C./minute. The temperature at the midpoint of the transition region is set as the glass transition temperature thereof. Additionally, when the modified conjugated diene polymer rubber is an oil extended product, the glass transition temperature is the glass transition temperature of the modified conjugated diene polymer rubber in a state where the oil extension component (the oil) is not included.

In the present technology, the content of the modified conjugated diene polymer rubber is not less than 25% by weight of 100% by weight of the diene rubber, and is preferably from 30% to 90% by weight, more preferably from 40% to 80% by weight, and even more preferably from 50% to 70% by weight. If the content of the modified conjugated diene polymer rubber is less than 25% by weight of the diene rubber, affinity with the silica will worsen, and dispersibility thereof cannot be made excellent. If the content of the modified conjugated diene polymer rubber exceeds 90% by weight, there is a possibility that the wear resistance may decline.

With the rubber composition for use in tire treads of the present technology, the diene rubber preferably comprises a natural rubber and/or a butadiene rubber in addition to the modified conjugated diene polymer rubber in order to enhance the wear resistance to or beyond conventional levels. Three embodiments are presented herein as rubber composition for use in tire treads by which wear resistance is improved. In the first embodiment, 100% by weight of the diene rubber comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber and from 15% to 45% by weight of a natural rubber. In the second embodiment, 100% by weight of the diene rubber comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber and from 15% to 35% by weight of a butadiene rubber. In the third embodiment, 100% by weight of the diene rubber comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber, from 15% to 45% by weight of a natural rubber, and not more than 15% by weight of a butadiene rubber. In each of these embodiments, wear resistance can be improved while maintaining superior low rolling resistance and wet performance.

In the first embodiment in which wear resistance is improved, a natural rubber is included in addition to the modified conjugated diene polymer rubber. As a result, wear resistance can be improved while maintaining the low rolling resistance and the wet performance at high levels. A compounded amount of the natural rubber is preferably from 15% to 45% by weight of 100% by weight of the diene rubber, and is more preferably from 20% to 40% by weight. By configuring the compounded amount of the natural rubber to be not less than 15% by weight, wet grip performance and wear resistance can be enhanced. If the compounded amount of the natural rubber exceeds 45% by weight, there is a possibility that an excessively high degree of non-compatibility will be displayed, which may lead to insufficient compound strength, worsening of the wet grip performance, and lack of improvement of the wear resistance. Any natural rubber that is regularly used in rubber compositions for use in tires may be used.

In the second embodiment in which wear resistance is improved, a butadiene rubber is included in addition to the modified conjugated diene polymer rubber. As a result, wear resistance can be improved while maintaining low rolling resistance and wet performance at a high level. A compounded amount of the butadiene rubber is preferably from 15% to 35% by weight of 100% by weight of the diene rubber. By configuring the compounded amount of the butadiene rubber to be not less than 15% by weight, wear resistance can be enhanced. Additionally, by configuring the compounded amount of the butadiene rubber to be not more than 35% by weight, wet grip performance can be ensured and wear resistance can be enhanced. Any butadiene rubber that is regularly used in rubber compositions for use in tires may be used.

In the third embodiment in which wear resistance is improved, a natural rubber and a butadiene rubber are included in addition to the modified conjugated diene polymer rubber. As a result, wear resistance can be improved while maintaining low rolling resistance and wet performance at a high level. 100% by weight of the diene rubber preferably comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber, from 15% to 45% by weight of the natural rubber, and not more than 15% by weight of the butadiene rubber. The compounded amount of the natural rubber is preferably from 15% to 45% by weight of 100% by weight of the diene rubber, and is more preferably from 20% to 40% by weight. By configuring the compounded amount of the natural rubber to be not more than 15% by weight, wet grip performance and wear resistance can be enhanced. Moreover, the rolling resistance can be reduced. Additionally, by configuring the compounded amount of the natural rubber to be not more than 45% by weight, wet grip performance can be ensured and low rolling resistance and wear resistance can be enhanced. Any natural rubber that is regularly used in rubber compositions for use in tires may be used.

In the third embodiment, the compounded amount of the butadiene rubber is preferably not more than 15% by weight and more preferably from 7% to 15% by weight of 100% by weight of the diene rubber. If the compounded amount of the butadiene rubber when used in combination with the natural rubber is 15% by weight, the wet performance and the wear resistance can be effectively enhanced.

Any butadiene rubber that is regularly used in rubber compositions for use in tires can be used. In the third embodiment particularly, the butadiene rubber preferably has a high molecular weight, a narrow molecular weight distribution, and few branches on the molecular chain. Preferably, the butadiene rubber has a weight-average molecular weight of from 700,000 to 900,000, a molecular weight distribution (Mw/Mn), calculated from the weight-average molecular weight (Mw) and the number average molecular weight (Mn) of from 1.5 to 3.0, and a toluene solution viscosity at 25° C. of from 300 to 1,000. In this case, the low rolling resistance and the wear resistance can be further enhanced while maintaining the wet performance.

The preferred molecular weight of the butadiene rubber is, in terms of weight-average molecular weight (Mw), preferably from 700,000 to 900,000 and more preferably from 760,000 to 850,000. If the weight-average molecular weight of the butadiene rubber is less than 700,000, the wear resistance and the strength of the rubber composition will be insufficient. If the weight-average molecular weight of the butadiene rubber exceeds 900,000, the viscosity of the rubber composition will increase and processability will be negatively affected. Moreover, dispersibility of a filler that contains silica will worsen.

Additionally, the molecular weight distribution (Mw/Mn) of the butadiene rubber, calculated from the weight-average molecular weight (Mw) and the number average molecular weight (Mn), is preferably from 1.5 to 3.0 and more preferably from 2.0 to 2.5. If the molecular weight distribution (Mw/Mn) of the butadiene rubber is less than 1.5, acquisition will be difficult and production cost will increase. If the molecular weight distribution (Mw/Mn) of the butadiene rubber exceeds 3.0, the wet performance cannot be sufficiently enhanced.

In the present technology, the weight-average molecular weight (Mw), the number average molecular weight (Mn), and the molecular weight distribution (Mw/Mn) of the butadiene rubber are measured via gel permeation chromatography (GPC), in terms of standard polystyrene.

The toluene solution viscosity at 25° C. of the butadiene rubber is preferably from 300 to 1,000 mPa·s and more preferably from 500 to 800 mPa·s. The toluene solution viscosity is an indicator of the linearity (amount of branches) of the molecular chain of the butadiene rubber. Higher toluene solution viscosity indicates less branches and a higher proportion of straight chain portions on the molecular chain, which leads to superior rubber strength (tensile breaking strength and breaking elongation). If the toluene solution viscosity of the butadiene rubber is less than 300 mPa·s, the wear resistance and the rubber strength of the rubber composition will be insufficient. If the toluene solution viscosity of the butadiene rubber exceeds 1,000 mPa·s, the viscosity of the rubber composition will increase and processability will be negatively affected. In the present technology, the toluene solution viscosity at 25° C. of the butadiene rubber is a measurement of the viscosity of a toluene solution containing 5% by weight of the butadiene rubber, at 25° C. using a Cannon-Fenske viscometer.

In the present technology, a diene rubber other than the modified conjugated diene polymer rubber, the natural rubber, and the butadiene rubber can be compounded as a rubber component. Examples of the other diene rubber include isoprene rubber, non-terminal-modified solution polymerization styrene butadiene rubber (S-SBR), emulsion polymerization styrene butadiene rubber (E-SBR), butyl rubber, halogenated butyl rubber, and the like. Of these, an emulsion polymerization styrene butadiene rubber is preferable. A single rubber may be used or multiple rubbers may be blended and used as the diene rubber. A content of the other diene rubber is not more than 75% by weight of 100% by weight of the diene rubber, and is preferably from 10% to 70% by weight and more preferably from 10% to 40% by weight.

In the present technology, a compounded amount of the filler comprising not less than 50% by weight of silica is from 25 to 65 parts by weight and preferably from 30 to 65 parts by weight per 100 parts by weight of the diene rubber. By configuring the compounded amount of the filler to be within this range, the low rolling resistance and wet performance of the rubber composition can be balanced at higher levels. Moreover, the wear resistance of the rubber composition can be ensured. If the compounded amount of the filler is less than 25 parts by weight, the wet performance cannot be ensured. Additionally, the wear resistance will decline. If the compounded amount of the filler exceeds 65 parts by weight, the rolling resistance will be negatively affected.

The content of the silica in 100% by weight of the filler is not less than 50% by weight and is preferably from 70% to 100% by weight. By configuring the content of the silica in the filler to be within this range, the low rolling resistance and the wet performance of the rubber composition are both achieved. Additionally, by compounding the modified conjugated diene polymer rubber, affinity with the silica is increased and dispersibility is enhanced. As a result, the effects of compounding silica are enhanced.

The silica may be any silica that is regularly used in rubber compositions for use in tire treads. Examples thereof include wet method silica, dry method silica, surface treated silica, and the like. Particle characteristics of the silica satisfy the four (1) to (4) characteristics described below.

(1) DBP absorption number: from 190 to 250 ml/100 g

The DBP absorption number of the silica is set to be from 190 to 250 ml/100 g. If the DBP absorption number is less than 190 ml/100 g, breaking strength will decline. If the DBP absorption number exceeds 250 ml/100 g, the dispersibility of the silica will decline and, as a result, the rolling resistance will worsen and the wear resistance will also worsen. The DBP absorption number of the silica is calculated in accordance with Oil Absorption Number Method A described in JIS K6217-4.

(2) Nitrogen specific surface area (N₂SA): from 194 to 225 m²/g

The nitrogen specific surface area (N₂SA) of the silica is set to be from 194 to 225 m²/g. If the N₂SA of the silica is less than 194 m²/g, the wet performance will worsen. If the N₂SA of the silica exceeds 225 m²/g, the dispersibility of the silica will decline and, as a result, the rolling resistance will worsen and the wear resistance will also worsen. The N₂SA of the silica is calculated in accordance with JIS K6217-2.

(3) CTAB specific surface area (CTAB): from 185 to 215 m²/g

The CTAB specific surface area (CTAB) of the silica is set to be from 185 to 215 m²/g. If the CTAB of the silica is less than 185 m²/g, the wet performance will worsen and the wear resistance will also worsen. If the CTAB of the silica exceeds 215 m²/g, the rolling resistance will worsen. The CTAB of the silica is calculated in accordance with JIS K6217-3.

(4) Ratio of the N₂SA to the CTAB (N₂SA/CTAB): from 0.9 to 1.4

The ratio of the N₂SA to the CTAB (N₂SA/CTAB) is set to be from 0.9 to 1.4. If the characteristic ratio of the silica (N₂SA/CTAB) is less than 0.9, reinforcement action will decline. If the characteristic ratio of the silica (N₂SA/CTAB) exceeds 1.4, the dispersibility of the silica will decline and the rolling resistance will worsen.

High specific surface area silica satisfying all of the particle characteristics (1) to (4) described above displays strong interaction between particle surfaces and poor affinity with diene rubber. As a result, it is difficult to obtain excellent dispersibility when simply compounded with diene rubber and the effects of improving the dynamic visco-elasticity characteristics such as the tan δ and the like cannot be sufficiently obtained. Additionally, sufficient improvement of the dispersibility of high specific surface area silica has not necessarily been obtained even when compounded together with a conventional terminal-modified styrene-butadiene rubber.

In contrast, in the present technology, the dispersibility of the high specific surface area silica can be improved by compounding the high specific surface area silica satisfying all of the particle characteristics (1) to (4) described above together with the modified conjugated diene polymer rubber described above. As such, the tan δ of both the modified conjugated diene polymer rubber and the high specific surface area silica are improved, which leads to the attainment of greater synergy. At the same time, the wear resistance of the rubber composition can be enhanced. Note that a single high specific surface area silica can be used as the silica. Alternatively, a high specific surface area silica and another silica that does not satisfy the particle characteristics (1) to (4) may be used together.

The silica to be used may be appropriately selected from commercially available products. Additionally, a silica obtained through a regular manufacturing method may be used.

In the rubber composition of the present technology, a silane coupling agent is preferably compounded together with the silica as such will lead to an improvement in the dispersibility of the silica and a further increase in the reinforcement action of the diene rubber. A compounded amount of the silane coupling agent is preferably from 3% to 20% by weight and more preferably from 5% to 15% by weight of the compounded amount of the silica. If the compounded amount of the silane coupling agent is less than 3% by weight of the weight of the silica, the effect of improving the dispersion of the silica cannot be sufficiently obtained. Additionally, if the compounded amount of the silane coupling agent exceeds 20% by weight, the silane coupling agents will polymerize, and the desired effects cannot be obtained.

The silane coupling agent is not particularly limited, but is preferably a sulfur-containing silane coupling agent. Examples thereof include bis-(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropyl benzothiazole tetrasulfide, γ-mercaptopropyltriethoxysilane, 3-octanoylthiopropyl triethoxysilane, and the like.

The rubber composition for use in tire treads of the present technology may also include other fillers other than the silica. Examples of such fillers other than the silica include, carbon black, clay, mica, talc, calcium carbonate, aluminum hydroxide, aluminum oxide, titanium oxide, and the like. Among these, carbon black is preferable. This is because rubber strength can be increased by compounding other fillers, including carbon black. A content of the other fillers is not more than 50% by weight and preferably from 0% to 30% by weight of 100% by weight of the filler. If the content of the other fillers exceeds 50% by weight, the rolling resistance will worsen.

The rubber composition for use in tire treads may also include various compounding agents that are commonly used in rubber compositions for use in tire treads. Examples thereof include vulcanization or cross-linking agents, vulcanization accelerators, antiaging agents, plasticizers, processing aids, liquid polymers, terpene resins, thermosetting resins, and the like. These compounding agents can be kneaded by a common method to obtain a composition that can then be used for vulcanization or cross-linking. These compounding agents can be blended at conventional general amounts so long as the objects of the present technology are not hindered. The rubber composition for use in tire treads can be produced by mixing the above-mentioned components using a well-known rubber kneading machine such as a Banbury mixer, a kneader, an open roll, or the like.

The rubber composition for use in tire treads of the present technology can be advantageously used in pneumatic tires. The low rolling resistance and the wet performance can be enhanced to or beyond conventional levels via a pneumatic tire in which this rubber composition is used in the tread portion. Additionally, with a pneumatic tire in which the rubber composition that includes the natural rubber and/or the butadiene rubber, wear resistance can be enhanced to or beyond conventional levels while superior low rolling resistance and wet performance are maintained.

The present technology is further described below by examples. However, the scope of the present technology is not limited to these examples.

EXAMPLES Working Examples 1 to 7

24 types of rubber compositions for use in tire treads were prepared according to the formulations shown in Tables 1 to 3 (Working Examples 1 to 7 and Comparative Examples 1 to 17). The shared components shown in Table 4 (with the exception of the sulfur and the vulcanization accelerator) were compounded with the rubber compositions and the mixtures were kneaded in a 1.8 L sealed mixed for five minutes at 160° C. Then, the mixtures were extruded as master batches, to which the sulfur and the vulcanization accelerator were added. Thereafter, the master batches were kneaded on an open roll. Note that in Tables 1 to 3, in cases where the SBR comprises an extension oil, the compounded amount of the SBR comprising this extension oil is noted along with the NET compounded amount of the SBR without the oil in parentheses. Additionally, the contents of the shared components shown in Table 4 are parts by weight compounded per 100 parts by weight of the diene rubber shown in Tables 1 to 3 (NET rubber content).

Vulcanized rubber samples were fabricated by pressure vulcanizing the obtained 24 types of rubber compositions for use in tire treads in a mold having a predetermined shape for 20 minutes at 160° C. Then, the rolling resistance (tan δ at 60° C.) of the samples was measured according to the methods described below.

Rolling Resistance: tan δ (60° C.)

The rolling resistance of the obtained vulcanized rubber samples was evaluated based on the loss tangent tan δ (60° C.), which is a known indicator of rolling resistance. The tan δ (60° C.) was measured using a viscoelasticity spectrometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) under the following conditions: 10% initial distortion, ±2% amplitude, 20 Hz frequency, and 60° C. temperature. The results of the measuring were indexed and recorded in Tables 1 to 3, with the index value of Comparative Example 1 being 100. Smaller index values indicate smaller tan δ (60° C.) and lower heat build-up, which in turn indicates lower rolling resistance and superior fuel consumption performance when used in a pneumatic tire. Next, sets of four pneumatic tires having the tire structure depicted in FIG. 1 and a tire size of 225/50R17 were fabricated. In each of the sets of four tires, one of the 24 types of rubber compositions for use in tire treads described above was used in the tread portion. The wet performance of each of the obtained 24 types of pneumatic tires was evaluated according to the method described below.

Wet Performance

The pneumatic tires were assembled on a wheel having a rim size of 7×J, and mounted on a 2.5 L class test vehicle (made in Japan). The pneumatic tires were inflated to an air pressure of 230 kPa and the test vehicle was driven on a 2.6 km circuit wet road surface test course. The wet performance while driving was scored based on sensory evaluation performed by three experienced evaluators. The results of the evaluation were indexed and recorded in Tables 1 to 3, with the index value of Comparative Example 1 being 100. Larger index values indicate superior wet performance on wet road surfaces.

TABLE 1 Com- Com- Com- parative parative parative Working Example 1 Example 2 Example 3 Example 1 Modified pbw 62.5 62.5 S-SBR 1 (50.0) (50.0) Modified pbw 50.0 50.0 S-SBR 2 E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0) (50.0) NR pbw Silica 1 pbw 40 40 Silica 2 pbw 40 40 Carbon black pbw 5 5 5 5 Silane coupling pbw 3.2 3.2 3.2 3.2 agent Oil pbw 12.5 12.5 0 0 tanδ (60° C.) Index 100 102 98 96 value Wet performance Index 100 97 105 110 value Working Working Working Working Example 2 Example 3 Example 4 Example 5 Modified pbw 62.5 37.5 87.5 62.5 S-SBR 1 (50.0) (30.0) (70.0) (50.0) Modified pbw S-SBR 2 E-SBR pbw 68.75 96.25 41.25 55 (50.0) (70.0) (30.0) (40.0) NR pbw 10.0 Silica 1 pbw Silica 2 pbw 60 40 40 40 Carbon black pbw 5 5 5 5 Silane coupling pbw 4.8 3.2 3.2 3.2 agent Oil pbw 0 0 2.5 3.75 tanδ (60° C.) Index 98 97 91 96 value Wet performance Index 133 107 115 111 value

TABLE 2 Comparative Comparative Comparative Comparative Example 4 Example 5 Example 6 Example 7 Modified S-SBR 1 pbw Modified S-SBR 3 pbw 60.0 (50.0) Modified S-SBR 4 pbw 50.0 Modified S-SBR 5 pbw 50.0 S-SBR pbw 68.75 (50.0) E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0) (50.0) Silica 2 pbw 40 40 40 40 Carbon black pbw 5 5 5 5 Silane coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 2.5 12.5 12.5 0 tanδ (60° C.) Index 99 98 100 100 value Wet performance Index 101 103 102 105 value Comparative Comparative Comparative Comparative Example 8 Example 9 Example 10 Example 11 Modified S-SBR 1 pbw 25.0 62.5 62.5 62.5 (20.0) (50.0) (50.0) (50.0) Modified S-SBR 3 pbw Modified S-SBR 4 pbw Modified S-SBR 5 pbw S-SBR pbw E-SBR pbw 110.0 68.75 68.75 68.75 (80.0) (50.0) (50.0) (50.0) Silica 2 pbw 40 20 20 70 Carbon black pbw 5 25 3 5 Silane coupling agent pbw 3.2 1.6 1.6 5.6 Oil pbw 0 0 0 0 tanδ (60° C.) Index 107 104 88 112 value Wet performance Index 95 102 81 117 value

TABLE 3 Working Working Comparative Comparative Example 6 Example 7 Example 12 Example 13 Modified S-SBR 1 pbw Modified S-SBR 6 pbw 68.75 (50.0) Modified S-SBR 7 pbw 68.75 (50.0) Modified S-SBR 8 pbw 68.75 (50.0) Modified S-SBR 9 pbw 68.75 (50.0) Modified S-SBR 10 pbw Modified S-SBR 11 pbw E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0) (50.0) Silica 2 pbw 40 40 40 40 Silica 3 pbw Silica 4 pbw Carbon black pbw 5 5 5 5 Silane coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 0 0 0 0 tanδ (60° C.) Index 97 97 89 103 value Wet performance Index 110 109 102 117 value Comparative Comparative Comparative Comparative Example 14 Example 15 Example 16 Example 17 Modified S-SBR 1 pbw 62.5 62.5 (50.0) (50.0) Modified S-SBR 6 pbw Modified S-SBR 7 pbw Modified S-SBR 8 pbw Modified S-SBR 9 pbw Modified S-SBR 10 pbw 68.75 (50.0) Modified S-SBR 11 pbw 68.75 (50.0) E-SBR pbw 68.75 68.75 68.75 68.75 (50.0) (50.0) (50.0) (50.0) Silica 2 pbw 40 40 Silica 3 pbw 40 Silica 4 pbw 40 Carbon black pbw 5 5 5 5 Silane coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 0 0 0 0 tanδ (60° C.) Index 88 102 103 89 value Wet performance Index 98 116 116 97 value

The types of raw materials used in Tables 1 to 3 are described below.

-   -   Modified S-SBR 1: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer         rubber; aromatic vinyl unit content of 42% by weight; vinyl unit         content of 32% by weight; weight-average molecular weight (Mw)         of 750,000; Tg of −25° C.; oil extended product comprising 25         parts by weight of oil per 100 parts by weight of the rubber         component.         [Production Method of Modified S-SBR 1]

4533 g of cyclohexane, 338.9 g (3.254 mol) of styrene, 468.0 g (8.652 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.189 mL (1.271 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 5.061 mL (7.945 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.281 g (0.318 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 18.3 g (0.318 mmol) of a xylene solution containing 40% by weight of polyorganosiloxane A described below was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrated using an open roll and dried in a dryer. Thus, the modified S-SBR 1 was obtained.

Polyorganosiloxane A: Polyorganosiloxane having the structure of the general formula (I), wherein m=80, n=0, k=120, X¹, X⁴, R¹ to R³, and R⁵ to R⁸ are each methyl groups (—CH₃), and X² is a hydrocarbon group expressed by formula (VIII) below.

-   -   Modified S-SBR 2: Terminal-modified solution polymerization         styrene butadiene rubber; aromatic vinyl unit content of 20% by         weight; vinyl unit content of 67%; weight-average molecular         weight (Mw) of 510,000; Tg of −25° C.; Nipol NS616 (Zeon         Corporation); non-oil extended product.     -   Modified S-SBR 3: Terminal-modified solution polymerization         styrene butadiene rubber; aromatic vinyl unit content of 35% by         weight; vinyl unit content of 48%; weight-average molecular         weight (Mw) of 450,000; Tg of −30° C.; SE0372 (manufactured by         Sumitomo Chemical Co., Ltd.); oil extended product comprising 20         parts by weight of oil per 100 parts by weight of the rubber         component.     -   Modified S-SBR 4: Terminal-modified solution polymerization         styrene butadiene rubber; aromatic vinyl unit content of 30% by         weight; vinyl unit content of 61%; weight-average molecular         weight (Mw) of 430,000; Tg of −27° C.; N207 (manufactured by         Asahi Kasei Corporation); non-oil extended product.     -   Modified S-SBR 5: Terminal-modified solution polymerization         styrene butadiene rubber; aromatic vinyl unit content of 42% by         weight; vinyl unit content of 35%; weight-average molecular         weight (Mw) of 440,000; Tg of −24° C.; Asaprene E10         (manufactured by Asahi Kasei Corporation); non-oil extended         product.     -   Modified S-SBR 6: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer rubber         formed from a polyorganosiloxane having the structure of the         general formula (II); aromatic vinyl unit content of 42% by         weight; vinyl unit content of 32%; weight-average molecular         weight (Mw) of 750,000; Tg of −25° C.; oil extended product         comprising 25 parts by weight of oil per 100 parts by weight of         the rubber component.         [Production Method of Modified S-SBR 6]

4550 g of cyclohexane, 341.1 g (3.275 mol) of styrene, 459.9 g (8.502 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.190 mL (1.277 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 5.062 mL (7.946 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.283 g (0.320 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 19.0 g (0.330 mmol) of a xylene solution containing 40% by weight of polyorganosiloxane B described below was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrate using an open roll and dried in a dryer. Thus, the modified S-SBR 6 was obtained.

Polyorganosiloxane B: Polyorganosiloxane having the structure of the general formula (II), wherein R⁹ to R¹⁶ are each methyl groups (—CH₃), and X⁵ to X⁸ are each hydrocarbon groups expressed by the formula (VIII).

-   -   Modified S-SBR 7: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer rubber         formed from a polyorganosiloxane having the structure of the         general formula (III); aromatic vinyl unit content of 41% by         weight; vinyl unit content of 32%; weight-average molecular         weight (Mw) of 750,000; Tg of −25° C.; oil extended product         comprising 25 parts by weight of oil per 100 parts by weight of         the rubber component.         [Production Method of Modified S-SBR 7]

4542 g of cyclohexane, 339.2 g (3.257 mol) of styrene, 462.8 g (8.556 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.188 mL (1.264 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 5.059 mL (7.942 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.283 g (0.320 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 19.2 g (0.333 mmol) of a xylene solution containing 40% by weight of polyorganosiloxane C described below was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrate using an open roll and dried in a dryer. Thus, the modified S-SBR 7 was obtained.

Polyorganosiloxane C: Polyorganosiloxane having the structure of the general formula (III), wherein s=2, R¹⁷ to R¹⁹ are each methyl groups (—CH₃), and X⁹ to X¹¹ are each hydrocarbon groups expressed by the formula (VIII).

-   -   Modified S-SBR 8: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer rubber         formed from a polyorganosiloxane having the structure of the         general formula (I); aromatic vinyl unit content of 34% by         weight; vinyl unit content of 34%; weight-average molecular         weight (Mw) of 760,000; Tg of −33° C.; oil extended product         comprising 25 parts by weight of oil per 100 parts by weight of         the rubber component.         [Production Method of Modified S-SBR 8]

4541 g of cyclohexane, 277.6 g (2.665 mol) of styrene, 523.1 g (9.671 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.175 mL (1.178 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 4.984 mL (7.824 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.273 g (0.327 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 18.1 g (0.314 mmol) of a xylene solution containing 40% by weight of the polyorganosiloxane A described above was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrate using an open roll and dried in a dryer. Thus, the modified S-SBR 8 was obtained.

-   -   Modified S-SBR 9: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer rubber         formed from a polyorganosiloxane having the structure of the         general formula (I); aromatic vinyl unit content of 49% by         weight; vinyl unit content of 28%; weight-average molecular         weight (Mw) of 710,000; Tg of −17° C.; oil extended product         comprising 25 parts by weight of oil per 100 parts by weight of         the rubber component.         [Production Method of Modified S-SBR 9]

4536 g of cyclohexane, 401.0 g (3.850 mol) of styrene, 392.0 g (7.247 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and 0.201 mL (1.352 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 5.141 mL (8.071 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.279 g (0.320 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 18.6 g (0.323 mmol) of a xylene solution containing 40% by weight of the polyorganosiloxane A described above was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrate using an open roll and dried in a dryer. Thus, the modified S-SBR 9 was obtained.

-   -   Modified S-SBR 10: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer rubber         formed from a polyorganosiloxane having the structure of the         general formula (I); aromatic vinyl unit content of 41% by         weight; vinyl unit content of 17%; weight-average molecular         weight (Mw) of 740,000; Tg of −37° C.; oil extended product         comprising 25 parts by weight of oil per 100 parts by weight of         the rubber component.         [Production Method of Modified S-SBR 10]

4542 g of cyclohexane, 339.2 g (3.257 mol) of styrene, 462.8 g (8.556 mol) of butadiene, 20.0 g (0.294 mol) of isoprene and 0.0376 mL (0.253 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 5.059 mL (7.942 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.280 g (0.331 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 18.8 g (0.326 mmol) of a xylene solution containing 40% by weight of the polyorganosiloxane A described above was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrate using an open roll and dried in a dryer. Thus, the modified S-SBR 10 was obtained.

-   -   Modified S-SBR 11: Terminal-modified solution polymerization         styrene butadiene rubber prepared according to the production         method described below; modified conjugated diene polymer rubber         formed from a polyorganosiloxane having the structure of the         general formula (I); aromatic vinyl unit content of 39% by         weight; vinyl unit content of 40%; weight-average molecular         weight (Mw) of 750,000; Tg of −21° C.; oil extended product         comprising 25 parts by weight of oil per 100 parts by weight of         the rubber component.         [Production Method of Modified S-SBR 11]

4543 g of cyclohexane, 319.8 g (3.071 mol) of styrene, 480.1 g (8.876 mol) of butadiene, 20.0 g (0.294 mol) of isoprene and 0.217 mL (1.462 mmol) of N,N,N′,N′-tetramethylethylenediamine were added to a nitrogen-purged autoclave reaction vessel having an internal capacity of 10 L. Then, mixing was begun. After adjusting the temperature of the matter in the reaction vessel to 50° C., 5.141 mL (8.0714 mmol) of n-butyllithium was added. After the rate of polymerization/conversion reached approximately 100%, 12.0 g more of isoprene was added and the mixture was reacted for five minutes. Then, 0.279 g (0.320 mmol) of a toluene solution containing 40% by weight of 1,6-bis(trichlorosilyl)hexane was added and the mixture was reacted for 30 minutes. Furthermore, 18.6 g (0.323 mmol) of a xylene solution containing 40% by weight of the polyorganosiloxane A described above was added and the mixture was reacted for 30 minutes. 0.5 mL of methanol was added and the mixture was mixed for 30 minutes. A small amount of antiaging agent (IRGANOX 1520, manufactured by BASF) and 25 parts of Fukko Luella Ceramic 30 (manufactured by Nippon Oil Corporation) as an extension oil were added to the resulting polymer solution. Then, the solid rubber was recovered using a steam stripping process. The obtained solid rubber was dehydrate using an open roll and dried in a dryer. Thus, the modified S-SBR 11 was obtained.

-   -   S-SBR: Unmodified solution polymerization styrene butadiene         rubber; aromatic vinyl unit content of 41% by weight; vinyl unit         content of 25% by weight; weight-average molecular weight (Mw)         of 1,010,000; Tg of −30° C.; SLR6430 (manufactured by Dow         Chemical); oil extended product comprising 37.5 parts by weight         of oil per 100 parts by weight of the rubber component.     -   E-SBR: Emulsion polymerization styrene butadiene rubber;         aromatic vinyl unit content of 25% by weight; vinyl unit content         of 15% by weight; weight-average molecular weight (Mw) of         600,000; Tg of −52° C.; Nipol 1723 (manufactured by Zeon         Corporation); oil extended product comprising 37.5 parts by         weight of oil per 100 parts by weight of the rubber component.     -   NR: Natural rubber; RSS #3     -   Silica 1: Zeosil 1165 MP (manufactured by Rhodia); DBP         absorption number of 200 ml/100 g; nitrogen specific surface         area (N₂SA) of 160 m²/g; CTAB specific surface area (CTAB) of         159 m²/g; ratio of N₂SA to CTAB (N₂SA/CTAB) of 1.01.     -   Silica 2: Zeosil Premium 200 MP (manufactured by Rhodia); DBP         absorption number of 203 ml/100 g; nitrogen specific surface         area (N₂SA) of 200 m²/g; CTAB specific surface area (CTAB) of         197 m²/g; ratio of N₂SA to CTAB (N₂SA/CTAB) of 1.02.     -   Silica 3: Nipsil KQ (manufactured by Tosoh Silica Corporation);         DBP absorption number of 261 ml/100 g; nitrogen specific surface         area (N₂SA) of 235 m²/g; CTAB specific surface area (CTAB) of         214 m²/g; ratio of N₂SA to CTAB (N₂SA/CTAB) of 1.10.     -   Silica 4: Nipsil AQ (manufactured by Tosoh Silica Corporation);         DBP absorption number of 192 ml/100 g; nitrogen specific surface         area (N₂SA) of 210 m²/g; CTAB specific surface area (CTAB) of         160 m²/g; ratio of N₂SA to CTAB (N₂SA/CTAB) of 1.31     -   Carbon black: SEAST KH (manufactured by Tokai Carbon Co., Ltd.)     -   Silane coupling agent: Si69 (manufactured by Evonik Degussa         Industries)     -   Oil: Extract No. 4S (manufactured by Showa Shell Seikyu K.K.)

TABLE 4 Shared Formulation of the Rubber Compositions Zinc oxide 3 parts by weight Stearic acid 2 parts by weight Antiaging agent 2 parts by weight Wax 2 parts by weight Sulfur 1.8 parts by weight   Vulcanization 2 parts by weight accelerator 1 Vulcanization 1.5 parts by weight   accelerator 2

The types of raw materials used in Table 4 are described below.

-   -   Zinc oxide: Zinc Oxide #3 (manufactured by Seido Chemical         Industry Co., Ltd.)     -   Stearic acid: Beads Stearic Acid YR (manufactured by NOF Corp.)     -   Antiaging agent: Santoflex 6PPD (manufactured by Flexsys)     -   Wax: SANNOC (manufactured by Ouchi Shinko Chemical Industrial)     -   Sulfur: “Golden Flower” oil-treated sulfur powder (manufactured         by Tsurumi Chemical Industry Co., Ltd.)     -   Vulcanization Accelerator 1: Vulcanization accelerator CBS;         Nocceler CZ-G (manufactured by Ouchi Shinko Chemical Industrial         Co., Ltd.)     -   Vulcanization Accelerator 2: Vulcanization accelerator DPG;         Nocceler D (manufactured by Ouchi Shinko Chemical Industrial         Co., Ltd.)

As is clear from Tables 1 to 3, with the rubber compositions for use in tire treads of Working Examples 1 to 7, enhanced low rolling resistance (tan δ at 60° C.) and wet performance were confirmed.

With the rubber composition of Comparative Example 2, the silica 1 was used in place of the silica 2 of Comparative Example 1 and, as a result, the balance of both the low rolling resistance and the wet performance was negatively affected.

With the rubber composition of Comparative Example 3, the modified S-SBR 1 was used in place of the modified S-SBR 2 of Comparative Example 2 and, as a result, while the balance of the low rolling resistance and the wet performance was improved, both the low rolling resistance and the wet performance were inferior compared to the rubber compositions of Working Examples 1 to 7 because the N₂SA of the silica 1 was less than 194 m²/g and the CTAB specific surface area was less than 185 m²/g.

As is clear from Table 2, with the rubber composition of Comparative Example 4, the rolling resistance cannot be reduced because the aromatic vinyl unit content in the modified S-SBR 3 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. With the rubber composition of Comparative Example 5, the rolling resistance cannot be sufficiently reduced because the aromatic vinyl unit content in the modified S-SBR 4 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. With the rubber composition of Comparative Example 6, the rolling resistance cannot be reduced because the weight-average molecular weight of the modified S-SBR 5 is less than 600,000. With the rubber composition of Comparative Example 7, the rolling resistance cannot be reduced because the unmodified S-SBR is compounded in place of the modified conjugated diene polymer rubber.

With the rubber composition of Comparative Example 8, the rolling resistance is negatively affected because the compounded amount of the modified S-SBR 1 in the diene rubber is less than 25% by weight. Moreover, the wet performance is insufficient. With the rubber composition of Comparative Example 9, the rolling resistance cannot be reduced because the content of the silica in the filler is less than 50% by weight. With the rubber composition of Comparative Example 10, the wet performance is negatively affected because the compounded amount of the filler comprising the silica is less than 25 parts by weight. With the rubber composition of Comparative Example 11, the rolling resistance is negatively affected because the compounded amount of the filler comprising the silica exceeds 65 parts by weight.

As is clear from Table 3, with the rubber composition of Comparative Example 12, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 1 to 7 because the aromatic vinyl unit content in the modified S-SBR 8 is less than 38% by weight. With the rubber composition of Comparative Example 13, the rolling resistance cannot be reduced because the aromatic vinyl monomer unit content in the modified S-SBR 9 is greater than 48% by weight. With the rubber composition of Comparative Example 14, the wet performance is negatively affected because the vinyl unit content in the modified S-SBR 10 is less than 20% by weight. With the rubber composition of Comparative Example 15, the rolling resistance cannot be reduced because the vinyl unit content in the modified S-SBR 11 exceeds 35% by weight.

With the rubber composition of Comparative Example 16, the dispersibility of the silica decreases and the rolling resistance is negatively affected because the DBP of the silica 3 exceeds 250 ml/100 g and the N₂SA exceeds 225 m²/g. With the rubber composition of Comparative Example 17, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 1 to 7 because the CTAB of the silica 4 is less than 185 m²/g.

Working Examples 8 to 23

46 types of rubber compositions for use in tire treads were prepared according to the formulations shown in Tables 5 to 10 (Working Examples 8 to 23 and Comparative Examples 18 to 47). The shared components shown in Table 4 (with the exception of the sulfur and the vulcanization accelerator) above were compounded with the rubber compositions and the mixtures were kneaded in a 1.8 L sealed mixed for five minutes at 160° C. Then, the mixtures were extruded as master batches, to which the sulfur and the vulcanization accelerator were added. Thereafter, the master batches were kneaded on an open roll. Note that in Tables 5 to 10, in cases where the SBR comprises an extension oil, the compounded amount of the SBR comprising this extension oil is noted along with the NET compounded amount of the SBR without the oil in parentheses. Additionally, the contents of the shared components shown in Table 4 are parts by weight compounded per 100 parts by weight of the diene rubber shown in Tables 5 to 10 (NET rubber content).

Vulcanized rubber samples were fabricated by pressure vulcanizing the obtained 46 types of rubber compositions for use in tire treads in a mold having a predetermined shape for 20 minutes at 160° C. Then, the rolling resistance (tan δ at 60° C.) and wear resistance of the samples were measured according to the methods described below.

Rolling Resistance: tan δ (60° C.)

The rolling resistance of the obtained vulcanized rubber samples was evaluated based on the loss tangent tan δ (60° C.), which is a known indicator of rolling resistance. The tan δ (60° C.) was measured using a viscoelasticity spectrometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) under the following conditions: 10% initial distortion, ±2% amplitude, 20 Hz frequency, and 60° C. temperature. The results of the measuring were indexed and recorded in Tables 5 to 10, with the index value of Comparative Example 18 being 100. Smaller index values indicate smaller tan δ (60° C.) and lower heat build-up, which in turn indicates lower rolling resistance and superior fuel consumption performance when used in a pneumatic tire.

Wear Resistance

Lambourn abrasion of the obtained vulcanized rubber samples was measured in accordance with JIS K6264-2 using a Lambourn abrasion resistance test machine (manufactured by Iwamoto Quartz GlassLab Co., Ltd.) under the following conditions: 20° C. temperature, 15 N load, and 50% slip ratio. The results of the measuring were indexed and recorded in Tables 5 to 10, with the index value of Comparative Example 18 being 100. Larger index values indicate superior abrasion resistance.

Next, sets of four pneumatic tires having the tire structure depicted in FIG. 1 and a tire size of 225/50R17 were fabricated. In each of the sets of four tires, one of the 46 types of rubber compositions for use in tire treads described above was used in the tread portion. The wet performance of each of the obtained 46 types of pneumatic tires was measured and evaluated according to the method described below.

Wet Performance

The pneumatic tires were assembled on a wheel having a rim size of 7×J, and mounted on a 2.5 L class test vehicle (made in Japan). The pneumatic tires were inflated to an air pressure of 230 kPa and the test vehicle was driven on a 2.6 km circuit wet road surface test course. The wet performance while driving was scored based on sensory evaluation performed by three experienced evaluators. The results of the evaluations were indexed and recorded in Tables 5 to 10, with the index value of Comparative Example 18 being 100. Larger index values indicate superior wet performance on wet road surfaces.

TABLE 5 Comparative Comparative Working Working Example 18 Example 19 Example 8 Example 9 Modified S-SBR 1 pbw 87.5 87.5 (70.0) (70.0) Modified S-SBR 2 pbw 70.0 70.0 E-SBR pbw 41.25 41.25 41.25 (30.0) (30.0) (30.0) S-SBR pbw NR pbw 30.0 Silica 1 pbw 40 Silica 2 pbw 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 17.5 17.5 0 11.25 tanδ (60° C.) Index 100 102 96 94 value Wet performance Index 100 97 110 113 value Wear resistance Index 100 97 102 110 value Working Working Working Working Example 10 Example 11 Example 12 Example 13 Modified S-SBR 1 pbw 87.5 87.5 68.75 87.5 (70.0) (70.0) (55.0) (70.0) Modified S-SBR 2 pbw E-SBR pbw 13.75 (10.0) S-SBR pbw 13.75 (10.0) NR pbw 20.0 20.0 45.0 30.0 Silica 1 pbw Silica 2 pbw 40 40 40 65 CB pbw 5 5 5 0 Coupling agent pbw 3.2 3.2 3.2 5.7 Oil pbw 7.5 7.5 15 11.25 tanδ (60° C.) Index 95 94 95 96 value Wet performance Index 112 114 114 136 value Wear resistance Index 107 111 112 112 value

TABLE 6 Comparative Comparative Comparative Comparative Example 20 Example 21 Example 22 Example 23 Modified S-SBR 1 pbw Modified S-SBR 3 pbw 84.0 (70.0) Modified S-SBR 4 pbw 70.0 Modified S-SBR 5 pbw 70.0 S-SBR pbw 96.25 (70.0) NR pbw 30.0 30.0 30.0 30.0 Silica 2 pbw 40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 97 96 98 98 tanδ (60° C.) Index 97 96 98 98 value Wet performance Index 105 104 107 108 value Wear resistance Index 102 100 102 107 value Comparative Comparative Comparative Comparative Example 24 Example 25 Example 26 Example 27 Modified S-SBR 1 pbw 106.25 87.5 87.5 87.5 (85.0) (70.0) (70.0) (70.0) Modified S-SBR 3 pbw Modified S-SBR 4 pbw Modified S-SBR 5 pbw S-SBR pbw NR pbw 15.0 30.0 30.0 30.0 Silica 2 pbw 40 20 15 70 CB pbw 5 25 3 5 Coupling agent pbw 3.2 1.6 1.2 5.6 Oil pbw 96 102 84 107 tanδ (60° C.) Index 96 102 84 107 value Wet performance Index 108 106 79 137 value Wear resistance Index 102 112 67 115 value

TABLE 7 Working Working Comparative Comparative Example 14 Example 15 Example 28 Example 29 Modified S-SBR 1 pbw Modified S-SBR 6 pbw 87.5 (70.0) Modified S-SBR 7 pbw 87.5 (70.0) Modified S-SBR 8 pbw 87.5 (70.0) Modified S-SBR 9 pbw 87.5 (70.0) Modified S-SBR 10 pbw Modified S-SBR 11 pbw NR pbw 30.0 30.0 30.0 30.0 Silica 2 pbw 40 40 40 40 Silica 3 pbw Silica 4 pbw CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 11.25 11.25 11.25 11.25 tanδ (60° C.) Index 97 97 89 103 value Wet performance Index 110 109 103 117 value Wear resistance Index 102 102 93 93 value Comparative Comparative Comparative Comparative Example 30 Example 31 Example 32 Example 33 Modified S-SBR 1 pbw 87.5 87.5 (70.0) (70.0) Modified S-SBR 6 pbw Modified S-SBR 7 pbw Modified S-SBR 8 pbw Modified S-SBR 9 pbw Modified S-SBR 10 pbw 87.5 (70.0) Modified S-SBR 11 pbw 87.5 (70.0) NR pbw 30.0 30.0 30.0 30.0 Silica 2 pbw 40 40 Silica 3 pbw 40 Silica 4 pbw 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 11.25 11.25 11.25 11.25 tanδ (60° C.) Index 88 102 106 88 value Wet performance Index 102 116 116 100 value Wear resistance Index 93 93 96 95 value

TABLE 8 Working Working Working Working Example 16 Example 17 Example 18 Example 19 Modified S-SBR 1 pbw 87.5 87.5 81.25 100.0 (70.0) (70.0) (65.0) (80.0) Modified S-SBR 3 pbw E-SBR pbw 41.25 (30.0) BR1 pbw 30.0 25.0 20.0 NR pbw 10.0 Silica 2 pbw 40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 0 11.25 12.5 8.8 tanδ (60° C.) Index 96 92 93 94 value Wet performance Index 110 112 113 115 value Wear resistance Index 102 112 110 109 value Working Working Comparative Example 20 Example 21 Example 34 Modified S-SBR 1 pbw 87.5 87.5 (70.0) (70.0) Modified S-SBR 3 pbw 84.0 (70.0) E-SBR pbw BR1 pbw 30.0 30.0 30.0 NR pbw Silica 2 pbw 25 65 40 CB pbw 10 0 5 Coupling agent pbw 2.2 5.7 3.2 Oil pbw 11.25 11.25 14.75 tanδ (60° C.) Index 90 94 95 value Wet performance Index 107 133 104 value Wear resistance Index 111 114 104 value

TABLE 9 Comparative Comparative Comparative Comparative Example 35 Example 36 Example 37 Example 38 Modified S-SBR 1 pbw 106.25 (85.0) Modified S-SBR 4 pbw 70.0 Modified S-SBR 5 pbw 70.0 S-SBR pbw 96.25 (70.0) BR1 pbw 30.0 30.0 30.0 15.0 Silica 2 pbw 40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 28.75 28.75 2.5 7.5 tanδ (60° C.) Index 94 96 96 97 value Wet performance Index 103 106 107 116 value Wear resistance Index 102 104 109 105 value Comparative Comparative Comparative Example 39 Example 40 Example 41 Modified S-SBR 1 pbw 87.5 87.5 87.5 (70.0) (70.0) (70.0) Modified S-SBR 4 pbw Modified S-SBR 5 pbw S-SBR pbw BR1 pbw 30.0 30.0 30.0 Silica 2 pbw 20 15 70 CB pbw 25 3 5 Coupling agent pbw 1.6 1.2 5.6 Oil pbw 11.25 11.25 11.25 tanδ (60° C.) Index 102 82 105 value Wet performance Index 105 96 136 value Wear resistance Index 114 69 117 value

TABLE 10 Working Working Comparative Comparative Example 22 Example 23 Example 42 Example 43 Modified S-SBR 1 pbw Modified S-SBR 6 pbw 87.5 (70.0) Modified S-SBR 7 pbw 87.5 (70.0) Modified S-SBR 8 pbw 87.5 (70.0) Modified S-SBR 9 pbw 87.5 (70.0) Modified S-SBR pbw 10 Modified S-SBR pbw 11 BR1 pbw 30.0 30.0 30.0 30.0 Silica 2 pbw 40 40 40 40 Silica 3 pbw Silica 4 pbw CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 11.25 11.25 11.25 11.25 tanδ (60° C.) Index 97 97 89 103 value Wet performance Index 110 109 101 115 value Wear resistance Index 102 102 93 93 value Comparative Comparative Comparative Comparative Example 44 Example 45 Example 46 Example 47 Modified S-SBR 1 pbw 87.5 87.5 (70.0) (70.0) Modified S-SBR 6 pbw Modified S-SBR 7 pbw Modified S-SBR 8 pbw Modified S-SBR 9 pbw Modified S-SBR pbw 87.5 10 (70.0) Modified S-SBR pbw 87.5 11 (70.0) BR1 pbw 30.0 30.0 30.0 30.0 Silica 2 pbw 40 40 Silica 3 pbw 40 Silica 4 pbw 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 11.25 11.25 11.25 11.25 tanδ (60° C.) Index 88 102 105 88 value Wet performance Index 100 114 116 101 value Wear resistance Index 93 93 96 95 value

Note that the types of raw materials used in Tables 5 to 10, with the exception of those described below, are the same as those used in Tables 1 to 3.

-   -   BR1: Butadiene rubber; Nipol BR1220 (manufactured by Zeon         Corporation)

As is clear from Tables 5 to 10, with the rubber compositions for use in tire treads of Working Examples 8 to 23, enhanced low rolling resistance (tan δ at 60° C.), wet performance, and wear resistance were confirmed.

With the rubber composition of Comparative Example 19, the silica 1 was used in place of the silica 2 of Comparative Example 18 and, as a result, compared to Comparative Example 18, the low rolling resistance (tan δ at 60° C.), wet performance, and wear resistance were each negatively affected.

As is clear from Table 6, with the rubber composition of Comparative Example 20, the rolling resistance cannot be sufficiently reduced and the wet performance and the wear resistance cannot be sufficiently improved because the aromatic vinyl unit content in the modified S-SBR 3 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. With the rubber composition of Comparative Example 21, the rolling resistance cannot be sufficiently reduced and the wear resistance cannot be improved because the aromatic vinyl unit content in the modified S-SBR 4 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. Moreover, the wet performance cannot be sufficiently improved. With the rubber composition of Comparative Example 22, the rolling resistance cannot be sufficiently reduced and the wear resistance cannot be sufficiently improved because the weight-average molecular weight of the modified S-SBR 5 is less than 600,000. With the rubber composition of Comparative Example 23, the rolling resistance cannot be sufficiently reduced because the unmodified S-SBR was compounded in place of the modified conjugated diene polymer rubber. Additionally, the wet performance and the wear resistance are lower than the Working Examples 8 to 23.

With the rubber composition of Comparative Example 24, the effects of improving the wear resistance are relatively small because the content of the modified S-SBR 1 in the diene rubber is 85% by weight. With the rubber composition of Comparative Example 25, the rolling resistance cannot be reduced because the content of the silica in the filler is less than 50% by weight. Additionally, the wet performance cannot be sufficiently improved compared to that of Working Examples 8 to 23. With the rubber composition of Comparative Example 26, the wet performance and the wear resistance decrease because the compounded amount of the filler comprising the silica is less than 25 parts by weight. With the rubber composition of Comparative Example 27, the rolling resistance is negatively affected because the compounded amount of the filler comprising the silica exceeds 65 parts by weight.

As is clear from Table 7, with the rubber composition of Comparative Example 28, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 8 to 15 because the aromatic vinyl unit content in the modified S-SBR 8 is less than 38% by weight. Moreover, the wear resistance is negatively affected. With the rubber composition of Comparative Example 29, the glass transition temperature (Tg) of the conjugated diene polymer rubber rises and the rolling resistance cannot be reduced because the aromatic vinyl monomer unit content in the modified S-SBR 9 is greater than 48% by weight. Furthermore, the wear resistance is negatively affected. With the rubber composition of Comparative Example 30, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 8 to 15 because the vinyl unit content in the modified S-SBR 10 is less than 20% by weight. Additionally, the wear resistance is negatively affected because the effects of increasing the strength cannot be sufficiently obtained. With the rubber composition of Comparative Example 31, the rolling resistance cannot be reduced and the wear resistance is negatively affected due to a decline in strength because the vinyl unit content in the modified S-SBR 11 exceeds 35% by weight.

With the rubber composition of Comparative Example 32, the dispersibility of the silica decreases, the rolling resistance is negatively affected, and the wear resistance is negatively affected because the DBP of the silica 3 exceeds 250 ml/100 g and the N₂SA exceeds 225 m²/g. With the rubber composition of Comparative Example 33, the effects of improving the wet performance cannot be obtained and, furthermore, the wear resistance is negatively affected because the CTAB of the silica 4 is less than 185 m²/g.

As is clear from Table 8, with the rubber composition of Comparative Example 34, the wet performance and the wear resistance cannot be sufficiently improved because the aromatic vinyl unit content in the modified S-SBR 3 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000.

As is clear from Table 9, with the rubber composition of Comparative Example 35, the wet performance and the wear resistance cannot be sufficiently improved because the aromatic vinyl unit content in the modified S-SBR 4 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. With the rubber composition of Comparative Example 36, the wet performance and the wear resistance cannot be sufficiently improved because the weight-average molecular weight of the modified S-SBR 5 is less than 600,000. With the rubber composition of Comparative Example 37, the balance of the rolling resistance and the wet performance is inferior compared to the Working Examples because the unmodified S-SBR is compounded in place of the modified conjugated diene polymer rubber. With the rubber composition of Comparative Example 38, the effects of improving the wear resistance are relatively small because the content of the modified S-SBR 1 in the diene rubber is 85% by weight. With the rubber composition of Comparative Example 39, the rolling resistance cannot be reduced because the content of the silica in the filler is less than 50% by weight. Additionally, the wet performance cannot be sufficiently improved compared to that of Working Examples 8 to 23. With the rubber composition of Comparative Example 40, the rolling resistance and the wear resistance are negatively affected because the compounded amount of the filler comprising the silica is less than 25 parts by weight. With the rubber composition of Comparative Example 41, the rolling resistance is negatively affected because the compounded amount of the filler comprising the silica exceeds 65 parts by weight.

As is clear from Table 10, with the rubber composition of Comparative Example 42, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 16 to 23 because the aromatic vinyl unit content in the modified S-SBR 8 is less than 38% by weight. Additionally, the wear resistance is negatively affected because the effects of increasing the strength cannot be sufficiently obtained. With the rubber composition of Comparative Example 43, the glass transition temperature (Tg) of the conjugated diene polymer rubber rises and the rolling resistance cannot be reduced because the aromatic vinyl unit content in the modified S-SBR 9 is greater than 48% by weight. Furthermore, the wear resistance is negatively affected. With the rubber composition of Comparative Example 44, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 16 to 23 because the vinyl unit content in the modified S-SBR 10 is less than 20% by weight. Additionally, the wear resistance is negatively affected because the effects of increasing the strength cannot be sufficiently obtained. With the rubber composition of Comparative Example 45, the rolling resistance cannot be reduced and the wear resistance is negatively affected due to a decline in strength because the vinyl unit content in the modified S-SBR 11 exceeds 35% by weight.

With the rubber composition of Comparative Example 46, the dispersibility of the silica decreases, the rolling resistance is negatively affected, and the wear resistance is negatively affected because the DBP of the silica 3 exceeds 250 ml/100 g and the N₂SA exceeds 225 m²/g. With the rubber composition of Comparative Example 47, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 16 to 23 and, furthermore, the wear resistance is negatively affected because the CTAB of the silica 4 is less than 185 m²/g.

Working Examples 24 to 31

23 types of rubber compositions for use in tire treads were prepared according to the formulations shown in Tables 11 to 13 (Working Examples 24 to 31 and Comparative Examples 48 to 62). The shared components shown in Table 4 (with the exception of the sulfur and the vulcanization accelerator) above were compounded with the rubber compositions and the mixtures were kneaded in a 1.8 L sealed mixed for five minutes at 160° C. Then, the mixtures were extruded as master batches, to which the sulfur and the vulcanization accelerator were added. Thereafter, the master batches were kneaded on an open roll. Note that in Tables 11 to 13, in cases where the SBR comprises an extension oil, the compounded amount of the SBR comprising this extension oil is noted along with the NET compounded amount of the SBR without the oil in parentheses. Additionally, the contents of the shared components shown in Table 4 are parts by weight compounded per 100 parts by weight of the diene rubber shown in Tables 11 to 13 (NET rubber content).

Vulcanized rubber samples were fabricated by pressure vulcanizing the obtained 23 types of rubber compositions for use in tire treads in a mold having a predetermined shape for 20 minutes at 160° C. Then, the rolling resistance (tan δ at 60° C.) and wear resistance of the samples was measured according to the methods described below.

Rolling Resistance: tan δ (60° C.)

The rolling resistance of the obtained vulcanized rubber samples was evaluated based on the loss tangent tan δ (60° C.), which is a known indicator of rolling resistance. The tan δ (60° C.) was measured using a viscoelasticity spectrometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) under the following conditions: 10% initial distortion, ±2% amplitude, 20 Hz frequency, and 60° C. temperature. The results of the measuring were indexed and recorded in Tables 11 to 13, with the index value of Comparative Example 48 being 100. Smaller index values indicate smaller tan δ (60° C.) and lower heat build-up, which in turn indicates lower rolling resistance and superior fuel consumption performance when used in a pneumatic tire.

Wear Resistance

Lambourn abrasion of the obtained vulcanized rubber samples was measured in accordance with JIS K6264-2 using a Lambourn abrasion resistance test machine (manufactured by Iwamoto Quartz GlassLab Co., Ltd.) under the following conditions: 20° C. temperature, 15 N load, and 50% slip ratio. The results of the measuring were indexed and recorded in Tables 11 to 13, with the index value of Comparative Example 48 being 100. Larger index values indicate superior wear resistance.

Next, sets of four pneumatic tires having the tire structure depicted in FIG. 1 and a tire size of 225/50R17 were fabricated. In each of the sets of four tires, one of the 23 types of rubber compositions for use in tire treads described above was used in the tread portion. The wet performance of each of the obtained 23 types of pneumatic tires was measured and evaluated according to the method described below.

Wet Performance

The pneumatic tires were assembled on a wheel having a rim size of 7×J, and mounted on a 2.5 L class test vehicle (made in Japan). The pneumatic tires were inflated to an air pressure of 230 kPa and the test vehicle was driven on a 2.6 km circuit wet road surface test course. The wet performance while driving was scored based on sensory evaluation performed by three experienced evaluators. The results of the measuring were indexed and recorded in Tables 11 to 13, with the index value of Comparative Example 48 being 100. Larger index values indicate superior wet performance on wet road surfaces.

TABLE 11 Comparative Comparative Working Working Example 48 Example 49 Example 24 Example 25 Modified S-SBR 1 pbw 68.75 68.75 (55.0) (55.0) Modified S-SBR 2 pbw 55.0 55.0 E-SBR pbw 61.88 (45.0) NR pbw 45.0 45.0 35.0 BR1 pbw 10.0 BR2 pbw Silica 1 pbw 40 Silica 2 pbw 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Terpene resin pbw Oil pbw 13.75 10.63 0 0 tanδ (60° C.) Index 100 102 96 94 value Wet performance Index 100 96 108 107 value Wear resistance Index 100 92 102 107 value Working Working Working Working Example 26 Example 27 Example 28 Example 29 Modified S-SBR 1 pbw 68.75 68.75 68.75 81.25 (55.0) (55.0) (55.0) (65.0) Modified S-SBR 2 pbw E-SBR pbw NR pbw 35.0 35.0 35.0 30.0 BR1 pbw 10.0 5.0 BR2 pbw 10.0 10.0 Silica 1 pbw Silica 2 pbw 40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Terpene resin pbw 5 5 Oil pbw 0 0 0 0 tanδ (60° C.) Index 93 94 93 94 value Wet performance Index 111 113 116 116 value Wear resistance Index 110 107 110 106 value

TABLE 12 Comparative Comparative Comparative Comparative Example 50 Example 51 Example 52 Example 53 Modified S-SBR 1 pbw Modified S-SBR 3 pbw 66.0 (55.0) Modified S-SBR 4 pbw 55.0 Modified S-SBR 5 pbw 55.0 S-SBR pbw 75.63 (55.0) NR pbw 35.0 35.0 35.0 35.0 BR1 pbw 10.0 10.0 10.0 10.0 Silica 2 pbw 40 40 40 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 2.75 13.75 13.75 0 tanδ (60° C.) Index 97 96 98 98 value Wet performance Index 99 98 101 94 value Wear resistance Index 99 97 99 92 value Comparative Comparative Comparative Example 54 Example 55 Example 56 Modified S-SBR 1 pbw 68.75 68.75 68.75 (55.0) (55.0) (55.0) Modified S-SBR 3 pbw Modified S-SBR 4 pbw Modified S-SBR 5 pbw S-SBR pbw NR pbw 35.0 35.0 35.0 BR1 pbw 10.0 10.0 10.0 Silica 2 pbw 20 15 70 CB pbw 25 3 5 Coupling agent pbw 1.6 1.2 5.6 Oil pbw 0 0 0 tanδ (60° C.) Index 102 84 107 value Wet performance Index 100 73 131 value Wear resistance Index 109 65 112 value

TABLE 13 Working Working Comparative Comparative Example 30 Example 31 Example 57 Example 58 Modified S-SBR 1 pbw Modified S-SBR 6 pbw 68.75 (55.0) Modified S-SBR 7 pbw 68.75 (55.0) Modified S-SBR 8 pbw 68.75 (55.0) Modified S-SBR 9 pbw 68.75 (55.0) Modified S-SBR 10 pbw Modified S-SBR 11 pbw NR pbw 35.0 35.0 35.0 35.0 BR1 pbw 10.0 10.0 10.0 10.0 Silica 2 pbw 40 40 40 40 Silica 3 pbw Silica 4 pbw CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 0 0 0 0 tanδ (60° C.) Index 95 95 88 103 value Wet performance Index 107 106 100 115 value Wear resistance Index 107 107 94 94 value Comparative Comparative Comparative Comparative Example 59 Example 60 Example 61 Example 62 Modified S-SBR 1 pbw 68.75 68.75 (55.0) (55.0) Modified S-SBR 6 pbw Modified S-SBR 7 pbw Modified S-SBR 8 pbw Modified S-SBR 9 pbw Modified S-SBR 10 pbw 68.75 (55.0) Modified S-SBR 11 pbw 68.75 (55.0) NR pbw 35.0 35.0 35.0 35.0 BR1 pbw 10.0 10.0 10.0 10.0 Silica 2 pbw 40 40 Silica 3 pbw 40 Silica 4 pbw 40 CB pbw 5 5 5 5 Coupling agent pbw 3.2 3.2 3.2 3.2 Oil pbw 0 0 0 0 tanδ (60° C.) Index 87 102 106 89 value Wet performance Index 99 114 114 99 value Wear resistance Index 94 94 96 95 value

Note that the types of raw materials used in Tables 11 to 13, with the exception of those described below, are the same as those used in Tables 1 to 10.

-   -   BR 1: Butadiene rubber; Nipol BR1220 (manufactured by Zeon         Corporation); weight-average molecular weight (Mw) of 490,000;         molecular weight distribution (Mw/Mn) of 2.7; toluene solution         viscosity of 60 mPa·s.     -   BR 2: Butadiene rubber; CB21 (manufactured by Lanxess);         weight-average molecular weight (Mw) of 770,000; molecular         weight distribution (Mw/Mn) of 2.4; toluene solution viscosity         of 600 mPa·s.     -   Terpene resin: Aromatic modified terpene resin; YS Resin TO125         (manufactured by Yasuhara Chemical Co., Ltd.)

As is clear from Tables 11 to 13, with the rubber compositions for use in tire treads of Working Examples 24 to 31, enhanced low rolling resistance (tan δ at 60° C.), wet performance, and wear resistance were confirmed. With the rubber composition of Comparative Example 49, the rolling resistance, the wet performance, and the wear resistance were each negatively affected compared to Comparative Example 48 because the E-SBR was compounded in place of the natural rubber of Comparative Example 48 and the silica 1 was used in place of the silica 2.

As is clear from Table 12, with the rubber composition of Comparative Example 50, the rolling resistance cannot be reduced and the wet performance and the wear resistance cannot be improved because the aromatic vinyl unit content in the modified S-SBR 3 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. With the rubber composition of Comparative Example 51, the wet performance and the wear resistance cannot be improved because the aromatic vinyl unit content in the modified S-SBR 4 is less than 38% by weight, the vinyl unit content exceeds 35% by weight, and the weight-average molecular weight is less than 600,000. With the rubber composition of Comparative Example 52, the rolling resistance cannot be sufficiently reduced because the weight-average molecular weight of the modified S-SBR 5 is less than 600,000. Moreover, the wet performance and the wear resistance cannot be improved. With the rubber composition of Comparative Example 53, the rolling resistance cannot be sufficiently reduced because the unmodified S-SBR was compounded in place of the modified conjugated diene polymer rubber. Moreover, the wet performance and the wear resistance decline.

With the rubber composition of Comparative Example 54, the rolling resistance cannot be reduced and the wet performance cannot be improved because the content of the silica in the filler is less than 50% by weight. Additionally, with the rubber composition of Comparative Example 55, the wet performance and the wear resistance cannot be improved because the compounded amount of the filler comprising the silica is less than 25 parts by weight. With the rubber composition of Comparative Example 56, the rolling resistance is negatively affected because the compounded amount of the filler comprising the silica exceeds 65 parts by weight.

As is clear from Table 13, with the rubber composition of Comparative Example 57, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 24 to 31 because the aromatic vinyl unit content in the modified S-SBR 8 is less than 38% by weight. Moreover, the wear resistance is negatively affected. With the rubber composition of Comparative Example 58, the glass transition temperature (Tg) of the conjugated diene polymer rubber rises and the rolling resistance cannot be reduced because the aromatic vinyl unit content in the modified S-SBR 9 is greater than 48% by weight. Furthermore, the wear resistance is negatively affected. With the rubber composition of Comparative Example 59, the effects of improving the wet performance are inferior compared to those achieved in Working Examples 24 to 31 because the vinyl unit content in the modified S-SBR 10 is less than 20% by weight. Additionally, the wear resistance is negatively affected because the effects of increasing the strength cannot be sufficiently obtained. With the rubber composition of Comparative Example 60, the rolling resistance cannot be reduced and the wear resistance is negatively affected due to a decline in strength because the vinyl unit content in the modified S-SBR 11 exceeds 35% by weight.

With the rubber composition of Comparative Example 61, the dispersibility of the silica decreases, the rolling resistance is negatively affected, and the wear resistance cannot be sufficiently improved because the DBP of the silica 3 exceeds 250 ml/100 g and the N₂SA exceeds 225 m²/g. With the rubber composition of Comparative Example 62, the effects of improving the wet performance cannot be obtained and, furthermore, the wear resistance is negatively affected because the CTAB of the silica 4 is less than 185 m²/g. 

What is claimed is:
 1. A rubber composition for use in tire treads comprising a diene rubber including 40 to 80% by weight of a modified conjugated diene polymer rubber and, per 100 parts by weight of the diene rubber, from 25 to 65 parts by weight of a filler; the filler comprising not less than 50% by weight of silica; the silica having a DBP absorption number of 190 to 250 ml/100 g, a nitrogen specific surface area (N₂SA) of 194 to 225 m²/g, and a CTAB specific surface area (CTAB) of 185 to 215 m²/g; a ratio of the N₂SA to the CTAB (N₂SA/CTAB) being from 0.9 to 1.4; the modified conjugated diene polymer rubber being obtained by copolymerizing a conjugated diene monomer unit and an aromatic vinyl monomer unit in a hydrocarbon solvent using an organic active metal compound as an initiator; a resulting active conjugated diene polymer chain thereof having a terminal modified group, obtained by reacting the active terminal of the polymer chain with at least one type of compound having a functional group that is reactable with the active terminal of the polymer chain; the terminal modified group having a functional group that interacts with the silica; and, in the modified conjugated diene polymer rubber, aromatic vinyl unit content being from 40% to 45% by weight, vinyl unit content being from 26% to 34%, and weight-average molecular weight being from 650,000 to 850,000.
 2. The rubber composition for use in tire treads according to claim 1, wherein 100% by weight of the diene rubber comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber and from 15% to 45% by weight of a natural rubber.
 3. The rubber composition for use in tire treads according to claim 2, wherein the compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain comprises at least one type of polyorganosiloxane compound selected from general formulae (I) to (III) below:

wherein R¹ to R⁸ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X¹ and X⁴ are identical or different and are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, alkyl groups having from 1 to 6 carbons, or aryl groups having from 6 to 12 carbons, X² is a group having a functional group that reacts with the active terminal of the active conjugated diene polymer chain, X³ is a group including from 2 to 20 repeating alkylene glycol units, a portion of the X³ moieties optionally being groups derived from groups including from 2 to 20 repeating alkylene glycol units, and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to 200;

wherein R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, and X⁵ to X⁸ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain;

wherein R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, and s is an integer from 1 to
 18. 4. The rubber composition for use in tire treads according to claim 1, wherein 100% by weight of the diene rubber comprises from 40% to 80% by weight of the modified conjugated diene polymer rubber and from 15% to 35% by weight of a butadiene rubber.
 5. The rubber composition for use in tire treads according to claim 4, wherein the compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain comprises at least one type of polyorganosiloxane compound selected from general formulae (I) to (III) below:

wherein R¹ to R⁸ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X¹ and X⁴ are identical or different and are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, alkyl groups having from 1 to 6 carbons, or aryl groups having from 6 to 12 carbons, X² is a group having a functional group that reacts with the active terminal of the active conjugated diene polymer chain, X³ is a group including from 2 to 20 repeating alkylene glycol units, a portion of the X³ moieties optionally being groups derived from groups including from 2 to 20 repeating alkylene glycol units, and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to 200;

wherein R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, and X⁵ to X⁸ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain;

wherein R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, and s is an integer from 1 to
 18. 6. The rubber composition for use in tire treads according to claim 1, wherein the compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain comprises at least one type of polyorganosiloxane compound selected from general formulae (I) to (III) below:

wherein R¹ to R⁸ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X¹ and X⁴ are identical or different and are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, alkyl groups having from 1 to 6 carbons, or aryl groups having from 6 to 12 carbons, X² is a group having a functional group that reacts with the active terminal of the active conjugated diene polymer chain, X³ is a group including from 2 to 20 repeating alkylene glycol units, a portion of the X³ moieties optionally being groups derived from groups including from 2 to 20 repeating alkylene glycol units, and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to 200;

wherein R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, and X⁵ to X⁸ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain;

wherein R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, and s is an integer from 1 to
 18. 7. The rubber composition for use in tire treads according to claim 1, wherein the compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain comprises at least one type of polyorganosiloxane compound according to general formula (I) below:

wherein R¹ to R⁸ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X¹ and X⁴ are identical or different and are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, alkyl groups having from 1 to 6 carbons, or aryl groups having from 6 to 12 carbons, X² is a group having a functional group that reacts with the active terminal of the active conjugated diene polymer chain, X³ is a group including from 2 to 20 repeating alkylene glycol units, a portion of the X³ moieties optionally being groups derived from groups including from 2 to 20 repeating alkylene glycol units, and m is an integer from 3 to 200, n is an integer from 0 to 200, and k is an integer from 0 to
 200. 8. The rubber composition for use in tire treads according to claim 1, wherein the compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain comprises at least one type of polyorganosiloxane compound according to general formula (II):

wherein R⁹ to R¹⁶ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, and X⁵ to X⁸ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain.
 9. The rubber composition for use in tire treads according to claim 1, wherein the compound comprising the functional group that is reactable with the active terminal of the active conjugated diene polymer chain comprises at least one type of polyorganosiloxane compound according to general formula (III) below:

wherein R¹⁷ to R¹⁹ are identical or different and are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, X⁹ to X¹¹ are groups having functional groups that react with the active terminal of the active conjugated diene polymer chain, and s is an integer from 1 to
 18. 10. A pneumatic tire comprising the rubber composition for use in tire treads described in claim
 1. 